view Documentation/block/biodoc.txt @ 897:329ea0ccb344

balloon: try harder to balloon up under memory pressure.

Currently if the balloon driver is unable to increase the guest's
reservation it assumes the failure was due to reaching its full
allocation, gives up on the ballooning operation and records the limit
it reached as the "hard limit". The driver will not try again until
the target is set again (even to the same value).

However it is possible that ballooning has in fact failed due to
memory pressure in the host and therefore it is desirable to keep
attempting to reach the target in case memory becomes available. The
most likely scenario is that some guests are ballooning down while
others are ballooning up and therefore there is temporary memory
pressure while things stabilise. You would not expect a well behaved
toolstack to ask a domain to balloon to more than its allocation nor
would you expect it to deliberately over-commit memory by setting
balloon targets which exceed the total host memory.

This patch drops the concept of a hard limit and causes the balloon
driver to retry increasing the reservation on a timer in the same
manner as when decreasing the reservation.

Also if we partially succeed in increasing the reservation
(i.e. receive less pages than we asked for) then we may as well keep
those pages rather than returning them to Xen.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
author Keir Fraser <keir.fraser@citrix.com>
date Fri Jun 05 14:01:20 2009 +0100 (2009-06-05)
parents 831230e53067
line source
1 Notes on the Generic Block Layer Rewrite in Linux 2.5
2 =====================================================
4 Notes Written on Jan 15, 2002:
5 Jens Axboe <axboe@suse.de>
6 Suparna Bhattacharya <suparna@in.ibm.com>
8 Last Updated May 2, 2002
9 September 2003: Updated I/O Scheduler portions
10 Nick Piggin <piggin@cyberone.com.au>
12 Introduction:
14 These are some notes describing some aspects of the 2.5 block layer in the
15 context of the bio rewrite. The idea is to bring out some of the key
16 changes and a glimpse of the rationale behind those changes.
18 Please mail corrections & suggestions to suparna@in.ibm.com.
20 Credits:
21 ---------
23 2.5 bio rewrite:
24 Jens Axboe <axboe@suse.de>
26 Many aspects of the generic block layer redesign were driven by and evolved
27 over discussions, prior patches and the collective experience of several
28 people. See sections 8 and 9 for a list of some related references.
30 The following people helped with review comments and inputs for this
31 document:
32 Christoph Hellwig <hch@infradead.org>
33 Arjan van de Ven <arjanv@redhat.com>
34 Randy Dunlap <rdunlap@xenotime.net>
35 Andre Hedrick <andre@linux-ide.org>
37 The following people helped with fixes/contributions to the bio patches
38 while it was still work-in-progress:
39 David S. Miller <davem@redhat.com>
42 Description of Contents:
43 ------------------------
45 1. Scope for tuning of logic to various needs
46 1.1 Tuning based on device or low level driver capabilities
47 - Per-queue parameters
48 - Highmem I/O support
49 - I/O scheduler modularization
50 1.2 Tuning based on high level requirements/capabilities
51 1.2.1 I/O Barriers
52 1.2.2 Request Priority/Latency
53 1.3 Direct access/bypass to lower layers for diagnostics and special
54 device operations
55 1.3.1 Pre-built commands
56 2. New flexible and generic but minimalist i/o structure or descriptor
57 (instead of using buffer heads at the i/o layer)
58 2.1 Requirements/Goals addressed
59 2.2 The bio struct in detail (multi-page io unit)
60 2.3 Changes in the request structure
61 3. Using bios
62 3.1 Setup/teardown (allocation, splitting)
63 3.2 Generic bio helper routines
64 3.2.1 Traversing segments and completion units in a request
65 3.2.2 Setting up DMA scatterlists
66 3.2.3 I/O completion
67 3.2.4 Implications for drivers that do not interpret bios (don't handle
68 multiple segments)
69 3.2.5 Request command tagging
70 3.3 I/O submission
71 4. The I/O scheduler
72 5. Scalability related changes
73 5.1 Granular locking: Removal of io_request_lock
74 5.2 Prepare for transition to 64 bit sector_t
75 6. Other Changes/Implications
76 6.1 Partition re-mapping handled by the generic block layer
77 7. A few tips on migration of older drivers
78 8. A list of prior/related/impacted patches/ideas
79 9. Other References/Discussion Threads
81 ---------------------------------------------------------------------------
83 Bio Notes
84 --------
86 Let us discuss the changes in the context of how some overall goals for the
87 block layer are addressed.
89 1. Scope for tuning the generic logic to satisfy various requirements
91 The block layer design supports adaptable abstractions to handle common
92 processing with the ability to tune the logic to an appropriate extent
93 depending on the nature of the device and the requirements of the caller.
94 One of the objectives of the rewrite was to increase the degree of tunability
95 and to enable higher level code to utilize underlying device/driver
96 capabilities to the maximum extent for better i/o performance. This is
97 important especially in the light of ever improving hardware capabilities
98 and application/middleware software designed to take advantage of these
99 capabilities.
101 1.1 Tuning based on low level device / driver capabilities
103 Sophisticated devices with large built-in caches, intelligent i/o scheduling
104 optimizations, high memory DMA support, etc may find some of the
105 generic processing an overhead, while for less capable devices the
106 generic functionality is essential for performance or correctness reasons.
107 Knowledge of some of the capabilities or parameters of the device should be
108 used at the generic block layer to take the right decisions on
109 behalf of the driver.
111 How is this achieved ?
113 Tuning at a per-queue level:
115 i. Per-queue limits/values exported to the generic layer by the driver
117 Various parameters that the generic i/o scheduler logic uses are set at
118 a per-queue level (e.g maximum request size, maximum number of segments in
119 a scatter-gather list, hardsect size)
121 Some parameters that were earlier available as global arrays indexed by
122 major/minor are now directly associated with the queue. Some of these may
123 move into the block device structure in the future. Some characteristics
124 have been incorporated into a queue flags field rather than separate fields
125 in themselves. There are blk_queue_xxx functions to set the parameters,
126 rather than update the fields directly
128 Some new queue property settings:
130 blk_queue_bounce_limit(q, u64 dma_address)
131 Enable I/O to highmem pages, dma_address being the
132 limit. No highmem default.
134 blk_queue_max_sectors(q, max_sectors)
135 Sets two variables that limit the size of the request.
137 - The request queue's max_sectors, which is a soft size in
138 in units of 512 byte sectors, and could be dynamically varied
139 by the core kernel.
141 - The request queue's max_hw_sectors, which is a hard limit
142 and reflects the maximum size request a driver can handle
143 in units of 512 byte sectors.
145 The default for both max_sectors and max_hw_sectors is
146 255. The upper limit of max_sectors is 1024.
148 blk_queue_max_phys_segments(q, max_segments)
149 Maximum physical segments you can handle in a request. 128
150 default (driver limit). (See 3.2.2)
152 blk_queue_max_hw_segments(q, max_segments)
153 Maximum dma segments the hardware can handle in a request. 128
154 default (host adapter limit, after dma remapping).
155 (See 3.2.2)
157 blk_queue_max_segment_size(q, max_seg_size)
158 Maximum size of a clustered segment, 64kB default.
160 blk_queue_hardsect_size(q, hardsect_size)
161 Lowest possible sector size that the hardware can operate
162 on, 512 bytes default.
164 New queue flags:
166 QUEUE_FLAG_CLUSTER (see 3.2.2)
167 QUEUE_FLAG_QUEUED (see 3.2.4)
170 ii. High-mem i/o capabilities are now considered the default
172 The generic bounce buffer logic, present in 2.4, where the block layer would
173 by default copyin/out i/o requests on high-memory buffers to low-memory buffers
174 assuming that the driver wouldn't be able to handle it directly, has been
175 changed in 2.5. The bounce logic is now applied only for memory ranges
176 for which the device cannot handle i/o. A driver can specify this by
177 setting the queue bounce limit for the request queue for the device
178 (blk_queue_bounce_limit()). This avoids the inefficiencies of the copyin/out
179 where a device is capable of handling high memory i/o.
181 In order to enable high-memory i/o where the device is capable of supporting
182 it, the pci dma mapping routines and associated data structures have now been
183 modified to accomplish a direct page -> bus translation, without requiring
184 a virtual address mapping (unlike the earlier scheme of virtual address
185 -> bus translation). So this works uniformly for high-memory pages (which
186 do not have a correponding kernel virtual address space mapping) and
187 low-memory pages.
189 Note: Please refer to DMA-mapping.txt for a discussion on PCI high mem DMA
190 aspects and mapping of scatter gather lists, and support for 64 bit PCI.
192 Special handling is required only for cases where i/o needs to happen on
193 pages at physical memory addresses beyond what the device can support. In these
194 cases, a bounce bio representing a buffer from the supported memory range
195 is used for performing the i/o with copyin/copyout as needed depending on
196 the type of the operation. For example, in case of a read operation, the
197 data read has to be copied to the original buffer on i/o completion, so a
198 callback routine is set up to do this, while for write, the data is copied
199 from the original buffer to the bounce buffer prior to issuing the
200 operation. Since an original buffer may be in a high memory area that's not
201 mapped in kernel virtual addr, a kmap operation may be required for
202 performing the copy, and special care may be needed in the completion path
203 as it may not be in irq context. Special care is also required (by way of
204 GFP flags) when allocating bounce buffers, to avoid certain highmem
205 deadlock possibilities.
207 It is also possible that a bounce buffer may be allocated from high-memory
208 area that's not mapped in kernel virtual addr, but within the range that the
209 device can use directly; so the bounce page may need to be kmapped during
210 copy operations. [Note: This does not hold in the current implementation,
211 though]
213 There are some situations when pages from high memory may need to
214 be kmapped, even if bounce buffers are not necessary. For example a device
215 may need to abort DMA operations and revert to PIO for the transfer, in
216 which case a virtual mapping of the page is required. For SCSI it is also
217 done in some scenarios where the low level driver cannot be trusted to
218 handle a single sg entry correctly. The driver is expected to perform the
219 kmaps as needed on such occasions using the __bio_kmap_atomic and bio_kmap_irq
220 routines as appropriate. A driver could also use the blk_queue_bounce()
221 routine on its own to bounce highmem i/o to low memory for specific requests
222 if so desired.
224 iii. The i/o scheduler algorithm itself can be replaced/set as appropriate
226 As in 2.4, it is possible to plugin a brand new i/o scheduler for a particular
227 queue or pick from (copy) existing generic schedulers and replace/override
228 certain portions of it. The 2.5 rewrite provides improved modularization
229 of the i/o scheduler. There are more pluggable callbacks, e.g for init,
230 add request, extract request, which makes it possible to abstract specific
231 i/o scheduling algorithm aspects and details outside of the generic loop.
232 It also makes it possible to completely hide the implementation details of
233 the i/o scheduler from block drivers.
235 I/O scheduler wrappers are to be used instead of accessing the queue directly.
236 See section 4. The I/O scheduler for details.
238 1.2 Tuning Based on High level code capabilities
240 i. Application capabilities for raw i/o
242 This comes from some of the high-performance database/middleware
243 requirements where an application prefers to make its own i/o scheduling
244 decisions based on an understanding of the access patterns and i/o
245 characteristics
247 ii. High performance filesystems or other higher level kernel code's
248 capabilities
250 Kernel components like filesystems could also take their own i/o scheduling
251 decisions for optimizing performance. Journalling filesystems may need
252 some control over i/o ordering.
254 What kind of support exists at the generic block layer for this ?
256 The flags and rw fields in the bio structure can be used for some tuning
257 from above e.g indicating that an i/o is just a readahead request, or for
258 marking barrier requests (discussed next), or priority settings (currently
259 unused). As far as user applications are concerned they would need an
260 additional mechanism either via open flags or ioctls, or some other upper
261 level mechanism to communicate such settings to block.
263 1.2.1 I/O Barriers
265 There is a way to enforce strict ordering for i/os through barriers.
266 All requests before a barrier point must be serviced before the barrier
267 request and any other requests arriving after the barrier will not be
268 serviced until after the barrier has completed. This is useful for higher
269 level control on write ordering, e.g flushing a log of committed updates
270 to disk before the corresponding updates themselves.
272 A flag in the bio structure, BIO_BARRIER is used to identify a barrier i/o.
273 The generic i/o scheduler would make sure that it places the barrier request and
274 all other requests coming after it after all the previous requests in the
275 queue. Barriers may be implemented in different ways depending on the
276 driver. For more details regarding I/O barriers, please read barrier.txt
277 in this directory.
279 1.2.2 Request Priority/Latency
281 Todo/Under discussion:
282 Arjan's proposed request priority scheme allows higher levels some broad
283 control (high/med/low) over the priority of an i/o request vs other pending
284 requests in the queue. For example it allows reads for bringing in an
285 executable page on demand to be given a higher priority over pending write
286 requests which haven't aged too much on the queue. Potentially this priority
287 could even be exposed to applications in some manner, providing higher level
288 tunability. Time based aging avoids starvation of lower priority
289 requests. Some bits in the bi_rw flags field in the bio structure are
290 intended to be used for this priority information.
293 1.3 Direct Access to Low level Device/Driver Capabilities (Bypass mode)
294 (e.g Diagnostics, Systems Management)
296 There are situations where high-level code needs to have direct access to
297 the low level device capabilities or requires the ability to issue commands
298 to the device bypassing some of the intermediate i/o layers.
299 These could, for example, be special control commands issued through ioctl
300 interfaces, or could be raw read/write commands that stress the drive's
301 capabilities for certain kinds of fitness tests. Having direct interfaces at
302 multiple levels without having to pass through upper layers makes
303 it possible to perform bottom up validation of the i/o path, layer by
304 layer, starting from the media.
306 The normal i/o submission interfaces, e.g submit_bio, could be bypassed
307 for specially crafted requests which such ioctl or diagnostics
308 interfaces would typically use, and the elevator add_request routine
309 can instead be used to directly insert such requests in the queue or preferably
310 the blk_do_rq routine can be used to place the request on the queue and
311 wait for completion. Alternatively, sometimes the caller might just
312 invoke a lower level driver specific interface with the request as a
313 parameter.
315 If the request is a means for passing on special information associated with
316 the command, then such information is associated with the request->special
317 field (rather than misuse the request->buffer field which is meant for the
318 request data buffer's virtual mapping).
320 For passing request data, the caller must build up a bio descriptor
321 representing the concerned memory buffer if the underlying driver interprets
322 bio segments or uses the block layer end*request* functions for i/o
323 completion. Alternatively one could directly use the request->buffer field to
324 specify the virtual address of the buffer, if the driver expects buffer
325 addresses passed in this way and ignores bio entries for the request type
326 involved. In the latter case, the driver would modify and manage the
327 request->buffer, request->sector and request->nr_sectors or
328 request->current_nr_sectors fields itself rather than using the block layer
329 end_request or end_that_request_first completion interfaces.
330 (See 2.3 or Documentation/block/request.txt for a brief explanation of
331 the request structure fields)
333 [TBD: end_that_request_last should be usable even in this case;
334 Perhaps an end_that_direct_request_first routine could be implemented to make
335 handling direct requests easier for such drivers; Also for drivers that
336 expect bios, a helper function could be provided for setting up a bio
337 corresponding to a data buffer]
339 <JENS: I dont understand the above, why is end_that_request_first() not
340 usable? Or _last for that matter. I must be missing something>
341 <SUP: What I meant here was that if the request doesn't have a bio, then
342 end_that_request_first doesn't modify nr_sectors or current_nr_sectors,
343 and hence can't be used for advancing request state settings on the
344 completion of partial transfers. The driver has to modify these fields
345 directly by hand.
346 This is because end_that_request_first only iterates over the bio list,
347 and always returns 0 if there are none associated with the request.
348 _last works OK in this case, and is not a problem, as I mentioned earlier
349 >
351 1.3.1 Pre-built Commands
353 A request can be created with a pre-built custom command to be sent directly
354 to the device. The cmd block in the request structure has room for filling
355 in the command bytes. (i.e rq->cmd is now 16 bytes in size, and meant for
356 command pre-building, and the type of the request is now indicated
357 through rq->flags instead of via rq->cmd)
359 The request structure flags can be set up to indicate the type of request
360 in such cases (REQ_PC: direct packet command passed to driver, REQ_BLOCK_PC:
361 packet command issued via blk_do_rq, REQ_SPECIAL: special request).
363 It can help to pre-build device commands for requests in advance.
364 Drivers can now specify a request prepare function (q->prep_rq_fn) that the
365 block layer would invoke to pre-build device commands for a given request,
366 or perform other preparatory processing for the request. This is routine is
367 called by elv_next_request(), i.e. typically just before servicing a request.
368 (The prepare function would not be called for requests that have REQ_DONTPREP
369 enabled)
371 Aside:
372 Pre-building could possibly even be done early, i.e before placing the
373 request on the queue, rather than construct the command on the fly in the
374 driver while servicing the request queue when it may affect latencies in
375 interrupt context or responsiveness in general. One way to add early
376 pre-building would be to do it whenever we fail to merge on a request.
377 Now REQ_NOMERGE is set in the request flags to skip this one in the future,
378 which means that it will not change before we feed it to the device. So
379 the pre-builder hook can be invoked there.
382 2. Flexible and generic but minimalist i/o structure/descriptor.
384 2.1 Reason for a new structure and requirements addressed
386 Prior to 2.5, buffer heads were used as the unit of i/o at the generic block
387 layer, and the low level request structure was associated with a chain of
388 buffer heads for a contiguous i/o request. This led to certain inefficiencies
389 when it came to large i/o requests and readv/writev style operations, as it
390 forced such requests to be broken up into small chunks before being passed
391 on to the generic block layer, only to be merged by the i/o scheduler
392 when the underlying device was capable of handling the i/o in one shot.
393 Also, using the buffer head as an i/o structure for i/os that didn't originate
394 from the buffer cache unecessarily added to the weight of the descriptors
395 which were generated for each such chunk.
397 The following were some of the goals and expectations considered in the
398 redesign of the block i/o data structure in 2.5.
400 i. Should be appropriate as a descriptor for both raw and buffered i/o -
401 avoid cache related fields which are irrelevant in the direct/page i/o path,
402 or filesystem block size alignment restrictions which may not be relevant
403 for raw i/o.
404 ii. Ability to represent high-memory buffers (which do not have a virtual
405 address mapping in kernel address space).
406 iii.Ability to represent large i/os w/o unecessarily breaking them up (i.e
407 greater than PAGE_SIZE chunks in one shot)
408 iv. At the same time, ability to retain independent identity of i/os from
409 different sources or i/o units requiring individual completion (e.g. for
410 latency reasons)
411 v. Ability to represent an i/o involving multiple physical memory segments
412 (including non-page aligned page fragments, as specified via readv/writev)
413 without unecessarily breaking it up, if the underlying device is capable of
414 handling it.
415 vi. Preferably should be based on a memory descriptor structure that can be
416 passed around different types of subsystems or layers, maybe even
417 networking, without duplication or extra copies of data/descriptor fields
418 themselves in the process
419 vii.Ability to handle the possibility of splits/merges as the structure passes
420 through layered drivers (lvm, md, evms), with minimal overhead.
422 The solution was to define a new structure (bio) for the block layer,
423 instead of using the buffer head structure (bh) directly, the idea being
424 avoidance of some associated baggage and limitations. The bio structure
425 is uniformly used for all i/o at the block layer ; it forms a part of the
426 bh structure for buffered i/o, and in the case of raw/direct i/o kiobufs are
427 mapped to bio structures.
429 2.2 The bio struct
431 The bio structure uses a vector representation pointing to an array of tuples
432 of <page, offset, len> to describe the i/o buffer, and has various other
433 fields describing i/o parameters and state that needs to be maintained for
434 performing the i/o.
436 Notice that this representation means that a bio has no virtual address
437 mapping at all (unlike buffer heads).
439 struct bio_vec {
440 struct page *bv_page;
441 unsigned short bv_len;
442 unsigned short bv_offset;
443 };
445 /*
446 * main unit of I/O for the block layer and lower layers (ie drivers)
447 */
448 struct bio {
449 sector_t bi_sector;
450 struct bio *bi_next; /* request queue link */
451 struct block_device *bi_bdev; /* target device */
452 unsigned long bi_flags; /* status, command, etc */
453 unsigned long bi_rw; /* low bits: r/w, high: priority */
455 unsigned int bi_vcnt; /* how may bio_vec's */
456 unsigned int bi_idx; /* current index into bio_vec array */
458 unsigned int bi_size; /* total size in bytes */
459 unsigned short bi_phys_segments; /* segments after physaddr coalesce*/
460 unsigned short bi_hw_segments; /* segments after DMA remapping */
461 unsigned int bi_max; /* max bio_vecs we can hold
462 used as index into pool */
463 struct bio_vec *bi_io_vec; /* the actual vec list */
464 bio_end_io_t *bi_end_io; /* bi_end_io (bio) */
465 atomic_t bi_cnt; /* pin count: free when it hits zero */
466 void *bi_private;
467 bio_destructor_t *bi_destructor; /* bi_destructor (bio) */
468 };
470 With this multipage bio design:
472 - Large i/os can be sent down in one go using a bio_vec list consisting
473 of an array of <page, offset, len> fragments (similar to the way fragments
474 are represented in the zero-copy network code)
475 - Splitting of an i/o request across multiple devices (as in the case of
476 lvm or raid) is achieved by cloning the bio (where the clone points to
477 the same bi_io_vec array, but with the index and size accordingly modified)
478 - A linked list of bios is used as before for unrelated merges (*) - this
479 avoids reallocs and makes independent completions easier to handle.
480 - Code that traverses the req list needs to make a distinction between
481 segments of a request (bio_for_each_segment) and the distinct completion
482 units/bios (rq_for_each_bio).
483 - Drivers which can't process a large bio in one shot can use the bi_idx
484 field to keep track of the next bio_vec entry to process.
485 (e.g a 1MB bio_vec needs to be handled in max 128kB chunks for IDE)
486 [TBD: Should preferably also have a bi_voffset and bi_vlen to avoid modifying
487 bi_offset an len fields]
489 (*) unrelated merges -- a request ends up containing two or more bios that
490 didn't originate from the same place.
492 bi_end_io() i/o callback gets called on i/o completion of the entire bio.
494 At a lower level, drivers build a scatter gather list from the merged bios.
495 The scatter gather list is in the form of an array of <page, offset, len>
496 entries with their corresponding dma address mappings filled in at the
497 appropriate time. As an optimization, contiguous physical pages can be
498 covered by a single entry where <page> refers to the first page and <len>
499 covers the range of pages (upto 16 contiguous pages could be covered this
500 way). There is a helper routine (blk_rq_map_sg) which drivers can use to build
501 the sg list.
503 Note: Right now the only user of bios with more than one page is ll_rw_kio,
504 which in turn means that only raw I/O uses it (direct i/o may not work
505 right now). The intent however is to enable clustering of pages etc to
506 become possible. The pagebuf abstraction layer from SGI also uses multi-page
507 bios, but that is currently not included in the stock development kernels.
508 The same is true of Andrew Morton's work-in-progress multipage bio writeout
509 and readahead patches.
511 2.3 Changes in the Request Structure
513 The request structure is the structure that gets passed down to low level
514 drivers. The block layer make_request function builds up a request structure,
515 places it on the queue and invokes the drivers request_fn. The driver makes
516 use of block layer helper routine elv_next_request to pull the next request
517 off the queue. Control or diagnostic functions might bypass block and directly
518 invoke underlying driver entry points passing in a specially constructed
519 request structure.
521 Only some relevant fields (mainly those which changed or may be referred
522 to in some of the discussion here) are listed below, not necessarily in
523 the order in which they occur in the structure (see include/linux/blkdev.h)
524 Refer to Documentation/block/request.txt for details about all the request
525 structure fields and a quick reference about the layers which are
526 supposed to use or modify those fields.
528 struct request {
529 struct list_head queuelist; /* Not meant to be directly accessed by
530 the driver.
531 Used by q->elv_next_request_fn
532 rq->queue is gone
533 */
534 .
535 .
536 unsigned char cmd[16]; /* prebuilt command data block */
537 unsigned long flags; /* also includes earlier rq->cmd settings */
538 .
539 .
540 sector_t sector; /* this field is now of type sector_t instead of int
541 preparation for 64 bit sectors */
542 .
543 .
545 /* Number of scatter-gather DMA addr+len pairs after
546 * physical address coalescing is performed.
547 */
548 unsigned short nr_phys_segments;
550 /* Number of scatter-gather addr+len pairs after
551 * physical and DMA remapping hardware coalescing is performed.
552 * This is the number of scatter-gather entries the driver
553 * will actually have to deal with after DMA mapping is done.
554 */
555 unsigned short nr_hw_segments;
557 /* Various sector counts */
558 unsigned long nr_sectors; /* no. of sectors left: driver modifiable */
559 unsigned long hard_nr_sectors; /* block internal copy of above */
560 unsigned int current_nr_sectors; /* no. of sectors left in the
561 current segment:driver modifiable */
562 unsigned long hard_cur_sectors; /* block internal copy of the above */
563 .
564 .
565 int tag; /* command tag associated with request */
566 void *special; /* same as before */
567 char *buffer; /* valid only for low memory buffers upto
568 current_nr_sectors */
569 .
570 .
571 struct bio *bio, *biotail; /* bio list instead of bh */
572 struct request_list *rl;
573 }
575 See the rq_flag_bits definitions for an explanation of the various flags
576 available. Some bits are used by the block layer or i/o scheduler.
578 The behaviour of the various sector counts are almost the same as before,
579 except that since we have multi-segment bios, current_nr_sectors refers
580 to the numbers of sectors in the current segment being processed which could
581 be one of the many segments in the current bio (i.e i/o completion unit).
582 The nr_sectors value refers to the total number of sectors in the whole
583 request that remain to be transferred (no change). The purpose of the
584 hard_xxx values is for block to remember these counts every time it hands
585 over the request to the driver. These values are updated by block on
586 end_that_request_first, i.e. every time the driver completes a part of the
587 transfer and invokes block end*request helpers to mark this. The
588 driver should not modify these values. The block layer sets up the
589 nr_sectors and current_nr_sectors fields (based on the corresponding
590 hard_xxx values and the number of bytes transferred) and updates it on
591 every transfer that invokes end_that_request_first. It does the same for the
592 buffer, bio, bio->bi_idx fields too.
594 The buffer field is just a virtual address mapping of the current segment
595 of the i/o buffer in cases where the buffer resides in low-memory. For high
596 memory i/o, this field is not valid and must not be used by drivers.
598 Code that sets up its own request structures and passes them down to
599 a driver needs to be careful about interoperation with the block layer helper
600 functions which the driver uses. (Section 1.3)
602 3. Using bios
604 3.1 Setup/Teardown
606 There are routines for managing the allocation, and reference counting, and
607 freeing of bios (bio_alloc, bio_get, bio_put).
609 This makes use of Ingo Molnar's mempool implementation, which enables
610 subsystems like bio to maintain their own reserve memory pools for guaranteed
611 deadlock-free allocations during extreme VM load. For example, the VM
612 subsystem makes use of the block layer to writeout dirty pages in order to be
613 able to free up memory space, a case which needs careful handling. The
614 allocation logic draws from the preallocated emergency reserve in situations
615 where it cannot allocate through normal means. If the pool is empty and it
616 can wait, then it would trigger action that would help free up memory or
617 replenish the pool (without deadlocking) and wait for availability in the pool.
618 If it is in IRQ context, and hence not in a position to do this, allocation
619 could fail if the pool is empty. In general mempool always first tries to
620 perform allocation without having to wait, even if it means digging into the
621 pool as long it is not less that 50% full.
623 On a free, memory is released to the pool or directly freed depending on
624 the current availability in the pool. The mempool interface lets the
625 subsystem specify the routines to be used for normal alloc and free. In the
626 case of bio, these routines make use of the standard slab allocator.
628 The caller of bio_alloc is expected to taken certain steps to avoid
629 deadlocks, e.g. avoid trying to allocate more memory from the pool while
630 already holding memory obtained from the pool.
631 [TBD: This is a potential issue, though a rare possibility
632 in the bounce bio allocation that happens in the current code, since
633 it ends up allocating a second bio from the same pool while
634 holding the original bio ]
636 Memory allocated from the pool should be released back within a limited
637 amount of time (in the case of bio, that would be after the i/o is completed).
638 This ensures that if part of the pool has been used up, some work (in this
639 case i/o) must already be in progress and memory would be available when it
640 is over. If allocating from multiple pools in the same code path, the order
641 or hierarchy of allocation needs to be consistent, just the way one deals
642 with multiple locks.
644 The bio_alloc routine also needs to allocate the bio_vec_list (bvec_alloc())
645 for a non-clone bio. There are the 6 pools setup for different size biovecs,
646 so bio_alloc(gfp_mask, nr_iovecs) will allocate a vec_list of the
647 given size from these slabs.
649 The bi_destructor() routine takes into account the possibility of the bio
650 having originated from a different source (see later discussions on
651 n/w to block transfers and kvec_cb)
653 The bio_get() routine may be used to hold an extra reference on a bio prior
654 to i/o submission, if the bio fields are likely to be accessed after the
655 i/o is issued (since the bio may otherwise get freed in case i/o completion
656 happens in the meantime).
658 The bio_clone() routine may be used to duplicate a bio, where the clone
659 shares the bio_vec_list with the original bio (i.e. both point to the
660 same bio_vec_list). This would typically be used for splitting i/o requests
661 in lvm or md.
663 3.2 Generic bio helper Routines
665 3.2.1 Traversing segments and completion units in a request
667 The macros bio_for_each_segment() and rq_for_each_bio() should be used for
668 traversing the bios in the request list (drivers should avoid directly
669 trying to do it themselves). Using these helpers should also make it easier
670 to cope with block changes in the future.
672 rq_for_each_bio(bio, rq)
673 bio_for_each_segment(bio_vec, bio, i)
674 /* bio_vec is now current segment */
676 I/O completion callbacks are per-bio rather than per-segment, so drivers
677 that traverse bio chains on completion need to keep that in mind. Drivers
678 which don't make a distinction between segments and completion units would
679 need to be reorganized to support multi-segment bios.
681 3.2.2 Setting up DMA scatterlists
683 The blk_rq_map_sg() helper routine would be used for setting up scatter
684 gather lists from a request, so a driver need not do it on its own.
686 nr_segments = blk_rq_map_sg(q, rq, scatterlist);
688 The helper routine provides a level of abstraction which makes it easier
689 to modify the internals of request to scatterlist conversion down the line
690 without breaking drivers. The blk_rq_map_sg routine takes care of several
691 things like collapsing physically contiguous segments (if QUEUE_FLAG_CLUSTER
692 is set) and correct segment accounting to avoid exceeding the limits which
693 the i/o hardware can handle, based on various queue properties.
695 - Prevents a clustered segment from crossing a 4GB mem boundary
696 - Avoids building segments that would exceed the number of physical
697 memory segments that the driver can handle (phys_segments) and the
698 number that the underlying hardware can handle at once, accounting for
699 DMA remapping (hw_segments) (i.e. IOMMU aware limits).
701 Routines which the low level driver can use to set up the segment limits:
703 blk_queue_max_hw_segments() : Sets an upper limit of the maximum number of
704 hw data segments in a request (i.e. the maximum number of address/length
705 pairs the host adapter can actually hand to the device at once)
707 blk_queue_max_phys_segments() : Sets an upper limit on the maximum number
708 of physical data segments in a request (i.e. the largest sized scatter list
709 a driver could handle)
711 3.2.3 I/O completion
713 The existing generic block layer helper routines end_request,
714 end_that_request_first and end_that_request_last can be used for i/o
715 completion (and setting things up so the rest of the i/o or the next
716 request can be kicked of) as before. With the introduction of multi-page
717 bio support, end_that_request_first requires an additional argument indicating
718 the number of sectors completed.
720 3.2.4 Implications for drivers that do not interpret bios (don't handle
721 multiple segments)
723 Drivers that do not interpret bios e.g those which do not handle multiple
724 segments and do not support i/o into high memory addresses (require bounce
725 buffers) and expect only virtually mapped buffers, can access the rq->buffer
726 field. As before the driver should use current_nr_sectors to determine the
727 size of remaining data in the current segment (that is the maximum it can
728 transfer in one go unless it interprets segments), and rely on the block layer
729 end_request, or end_that_request_first/last to take care of all accounting
730 and transparent mapping of the next bio segment when a segment boundary
731 is crossed on completion of a transfer. (The end*request* functions should
732 be used if only if the request has come down from block/bio path, not for
733 direct access requests which only specify rq->buffer without a valid rq->bio)
735 3.2.5 Generic request command tagging
737 Tag helpers
739 Block now offers some simple generic functionality to help support command
740 queueing (typically known as tagged command queueing), ie manage more than
741 one outstanding command on a queue at any given time.
743 blk_queue_init_tags(request_queue_t *q, int depth)
745 Initialize internal command tagging structures for a maximum
746 depth of 'depth'.
748 blk_queue_free_tags((request_queue_t *q)
750 Teardown tag info associated with the queue. This will be done
751 automatically by block if blk_queue_cleanup() is called on a queue
752 that is using tagging.
754 The above are initialization and exit management, the main helpers during
755 normal operations are:
757 blk_queue_start_tag(request_queue_t *q, struct request *rq)
759 Start tagged operation for this request. A free tag number between
760 0 and 'depth' is assigned to the request (rq->tag holds this number),
761 and 'rq' is added to the internal tag management. If the maximum depth
762 for this queue is already achieved (or if the tag wasn't started for
763 some other reason), 1 is returned. Otherwise 0 is returned.
765 blk_queue_end_tag(request_queue_t *q, struct request *rq)
767 End tagged operation on this request. 'rq' is removed from the internal
768 book keeping structures.
770 To minimize struct request and queue overhead, the tag helpers utilize some
771 of the same request members that are used for normal request queue management.
772 This means that a request cannot both be an active tag and be on the queue
773 list at the same time. blk_queue_start_tag() will remove the request, but
774 the driver must remember to call blk_queue_end_tag() before signalling
775 completion of the request to the block layer. This means ending tag
776 operations before calling end_that_request_last()! For an example of a user
777 of these helpers, see the IDE tagged command queueing support.
779 Certain hardware conditions may dictate a need to invalidate the block tag
780 queue. For instance, on IDE any tagged request error needs to clear both
781 the hardware and software block queue and enable the driver to sanely restart
782 all the outstanding requests. There's a third helper to do that:
784 blk_queue_invalidate_tags(request_queue_t *q)
786 Clear the internal block tag queue and readd all the pending requests
787 to the request queue. The driver will receive them again on the
788 next request_fn run, just like it did the first time it encountered
789 them.
791 Tag info
793 Some block functions exist to query current tag status or to go from a
794 tag number to the associated request. These are, in no particular order:
796 blk_queue_tagged(q)
798 Returns 1 if the queue 'q' is using tagging, 0 if not.
800 blk_queue_tag_request(q, tag)
802 Returns a pointer to the request associated with tag 'tag'.
804 blk_queue_tag_depth(q)
806 Return current queue depth.
808 blk_queue_tag_queue(q)
810 Returns 1 if the queue can accept a new queued command, 0 if we are
811 at the maximum depth already.
813 blk_queue_rq_tagged(rq)
815 Returns 1 if the request 'rq' is tagged.
817 Internal structure
819 Internally, block manages tags in the blk_queue_tag structure:
821 struct blk_queue_tag {
822 struct request **tag_index; /* array or pointers to rq */
823 unsigned long *tag_map; /* bitmap of free tags */
824 struct list_head busy_list; /* fifo list of busy tags */
825 int busy; /* queue depth */
826 int max_depth; /* max queue depth */
827 };
829 Most of the above is simple and straight forward, however busy_list may need
830 a bit of explaining. Normally we don't care too much about request ordering,
831 but in the event of any barrier requests in the tag queue we need to ensure
832 that requests are restarted in the order they were queue. This may happen
833 if the driver needs to use blk_queue_invalidate_tags().
835 Tagging also defines a new request flag, REQ_QUEUED. This is set whenever
836 a request is currently tagged. You should not use this flag directly,
837 blk_rq_tagged(rq) is the portable way to do so.
839 3.3 I/O Submission
841 The routine submit_bio() is used to submit a single io. Higher level i/o
842 routines make use of this:
844 (a) Buffered i/o:
845 The routine submit_bh() invokes submit_bio() on a bio corresponding to the
846 bh, allocating the bio if required. ll_rw_block() uses submit_bh() as before.
848 (b) Kiobuf i/o (for raw/direct i/o):
849 The ll_rw_kio() routine breaks up the kiobuf into page sized chunks and
850 maps the array to one or more multi-page bios, issuing submit_bio() to
851 perform the i/o on each of these.
853 The embedded bh array in the kiobuf structure has been removed and no
854 preallocation of bios is done for kiobufs. [The intent is to remove the
855 blocks array as well, but it's currently in there to kludge around direct i/o.]
856 Thus kiobuf allocation has switched back to using kmalloc rather than vmalloc.
858 Todo/Observation:
860 A single kiobuf structure is assumed to correspond to a contiguous range
861 of data, so brw_kiovec() invokes ll_rw_kio for each kiobuf in a kiovec.
862 So right now it wouldn't work for direct i/o on non-contiguous blocks.
863 This is to be resolved. The eventual direction is to replace kiobuf
864 by kvec's.
866 Badari Pulavarty has a patch to implement direct i/o correctly using
867 bio and kvec.
870 (c) Page i/o:
871 Todo/Under discussion:
873 Andrew Morton's multi-page bio patches attempt to issue multi-page
874 writeouts (and reads) from the page cache, by directly building up
875 large bios for submission completely bypassing the usage of buffer
876 heads. This work is still in progress.
878 Christoph Hellwig had some code that uses bios for page-io (rather than
879 bh). This isn't included in bio as yet. Christoph was also working on a
880 design for representing virtual/real extents as an entity and modifying
881 some of the address space ops interfaces to utilize this abstraction rather
882 than buffer_heads. (This is somewhat along the lines of the SGI XFS pagebuf
883 abstraction, but intended to be as lightweight as possible).
885 (d) Direct access i/o:
886 Direct access requests that do not contain bios would be submitted differently
887 as discussed earlier in section 1.3.
889 Aside:
891 Kvec i/o:
893 Ben LaHaise's aio code uses a slighly different structure instead
894 of kiobufs, called a kvec_cb. This contains an array of <page, offset, len>
895 tuples (very much like the networking code), together with a callback function
896 and data pointer. This is embedded into a brw_cb structure when passed
897 to brw_kvec_async().
899 Now it should be possible to directly map these kvecs to a bio. Just as while
900 cloning, in this case rather than PRE_BUILT bio_vecs, we set the bi_io_vec
901 array pointer to point to the veclet array in kvecs.
903 TBD: In order for this to work, some changes are needed in the way multi-page
904 bios are handled today. The values of the tuples in such a vector passed in
905 from higher level code should not be modified by the block layer in the course
906 of its request processing, since that would make it hard for the higher layer
907 to continue to use the vector descriptor (kvec) after i/o completes. Instead,
908 all such transient state should either be maintained in the request structure,
909 and passed on in some way to the endio completion routine.
912 4. The I/O scheduler
913 I/O scheduler, a.k.a. elevator, is implemented in two layers. Generic dispatch
914 queue and specific I/O schedulers. Unless stated otherwise, elevator is used
915 to refer to both parts and I/O scheduler to specific I/O schedulers.
917 Block layer implements generic dispatch queue in ll_rw_blk.c and elevator.c.
918 The generic dispatch queue is responsible for properly ordering barrier
919 requests, requeueing, handling non-fs requests and all other subtleties.
921 Specific I/O schedulers are responsible for ordering normal filesystem
922 requests. They can also choose to delay certain requests to improve
923 throughput or whatever purpose. As the plural form indicates, there are
924 multiple I/O schedulers. They can be built as modules but at least one should
925 be built inside the kernel. Each queue can choose different one and can also
926 change to another one dynamically.
928 A block layer call to the i/o scheduler follows the convention elv_xxx(). This
929 calls elevator_xxx_fn in the elevator switch (drivers/block/elevator.c). Oh,
930 xxx and xxx might not match exactly, but use your imagination. If an elevator
931 doesn't implement a function, the switch does nothing or some minimal house
932 keeping work.
934 4.1. I/O scheduler API
936 The functions an elevator may implement are: (* are mandatory)
937 elevator_merge_fn called to query requests for merge with a bio
939 elevator_merge_req_fn called when two requests get merged. the one
940 which gets merged into the other one will be
941 never seen by I/O scheduler again. IOW, after
942 being merged, the request is gone.
944 elevator_merged_fn called when a request in the scheduler has been
945 involved in a merge. It is used in the deadline
946 scheduler for example, to reposition the request
947 if its sorting order has changed.
949 elevator_dispatch_fn fills the dispatch queue with ready requests.
950 I/O schedulers are free to postpone requests by
951 not filling the dispatch queue unless @force
952 is non-zero. Once dispatched, I/O schedulers
953 are not allowed to manipulate the requests -
954 they belong to generic dispatch queue.
956 elevator_add_req_fn called to add a new request into the scheduler
958 elevator_queue_empty_fn returns true if the merge queue is empty.
959 Drivers shouldn't use this, but rather check
960 if elv_next_request is NULL (without losing the
961 request if one exists!)
963 elevator_former_req_fn
964 elevator_latter_req_fn These return the request before or after the
965 one specified in disk sort order. Used by the
966 block layer to find merge possibilities.
968 elevator_completed_req_fn called when a request is completed.
970 elevator_may_queue_fn returns true if the scheduler wants to allow the
971 current context to queue a new request even if
972 it is over the queue limit. This must be used
973 very carefully!!
975 elevator_set_req_fn
976 elevator_put_req_fn Must be used to allocate and free any elevator
977 specific storage for a request.
979 elevator_activate_req_fn Called when device driver first sees a request.
980 I/O schedulers can use this callback to
981 determine when actual execution of a request
982 starts.
983 elevator_deactivate_req_fn Called when device driver decides to delay
984 a request by requeueing it.
986 elevator_init_fn
987 elevator_exit_fn Allocate and free any elevator specific storage
988 for a queue.
990 4.2 Request flows seen by I/O schedulers
991 All requests seens by I/O schedulers strictly follow one of the following three
992 flows.
994 set_req_fn ->
996 i. add_req_fn -> (merged_fn ->)* -> dispatch_fn -> activate_req_fn ->
997 (deactivate_req_fn -> activate_req_fn ->)* -> completed_req_fn
998 ii. add_req_fn -> (merged_fn ->)* -> merge_req_fn
999 iii. [none]
1001 -> put_req_fn
1003 4.3 I/O scheduler implementation
1004 The generic i/o scheduler algorithm attempts to sort/merge/batch requests for
1005 optimal disk scan and request servicing performance (based on generic
1006 principles and device capabilities), optimized for:
1007 i. improved throughput
1008 ii. improved latency
1009 iii. better utilization of h/w & CPU time
1011 Characteristics:
1013 i. Binary tree
1014 AS and deadline i/o schedulers use red black binary trees for disk position
1015 sorting and searching, and a fifo linked list for time-based searching. This
1016 gives good scalability and good availablility of information. Requests are
1017 almost always dispatched in disk sort order, so a cache is kept of the next
1018 request in sort order to prevent binary tree lookups.
1020 This arrangement is not a generic block layer characteristic however, so
1021 elevators may implement queues as they please.
1023 ii. Merge hash
1024 AS and deadline use a hash table indexed by the last sector of a request. This
1025 enables merging code to quickly look up "back merge" candidates, even when
1026 multiple I/O streams are being performed at once on one disk.
1028 "Front merges", a new request being merged at the front of an existing request,
1029 are far less common than "back merges" due to the nature of most I/O patterns.
1030 Front merges are handled by the binary trees in AS and deadline schedulers.
1032 iii. Plugging the queue to batch requests in anticipation of opportunities for
1033 merge/sort optimizations
1035 This is just the same as in 2.4 so far, though per-device unplugging
1036 support is anticipated for 2.5. Also with a priority-based i/o scheduler,
1037 such decisions could be based on request priorities.
1039 Plugging is an approach that the current i/o scheduling algorithm resorts to so
1040 that it collects up enough requests in the queue to be able to take
1041 advantage of the sorting/merging logic in the elevator. If the
1042 queue is empty when a request comes in, then it plugs the request queue
1043 (sort of like plugging the bottom of a vessel to get fluid to build up)
1044 till it fills up with a few more requests, before starting to service
1045 the requests. This provides an opportunity to merge/sort the requests before
1046 passing them down to the device. There are various conditions when the queue is
1047 unplugged (to open up the flow again), either through a scheduled task or
1048 could be on demand. For example wait_on_buffer sets the unplugging going
1049 (by running tq_disk) so the read gets satisfied soon. So in the read case,
1050 the queue gets explicitly unplugged as part of waiting for completion,
1051 in fact all queues get unplugged as a side-effect.
1053 Aside:
1054 This is kind of controversial territory, as it's not clear if plugging is
1055 always the right thing to do. Devices typically have their own queues,
1056 and allowing a big queue to build up in software, while letting the device be
1057 idle for a while may not always make sense. The trick is to handle the fine
1058 balance between when to plug and when to open up. Also now that we have
1059 multi-page bios being queued in one shot, we may not need to wait to merge
1060 a big request from the broken up pieces coming by.
1062 Per-queue granularity unplugging (still a Todo) may help reduce some of the
1063 concerns with just a single tq_disk flush approach. Something like
1064 blk_kick_queue() to unplug a specific queue (right away ?)
1065 or optionally, all queues, is in the plan.
1067 4.4 I/O contexts
1068 I/O contexts provide a dynamically allocated per process data area. They may
1069 be used in I/O schedulers, and in the block layer (could be used for IO statis,
1070 priorities for example). See *io_context in block/ll_rw_blk.c, and as-iosched.c
1071 for an example of usage in an i/o scheduler.
1074 5. Scalability related changes
1076 5.1 Granular Locking: io_request_lock replaced by a per-queue lock
1078 The global io_request_lock has been removed as of 2.5, to avoid
1079 the scalability bottleneck it was causing, and has been replaced by more
1080 granular locking. The request queue structure has a pointer to the
1081 lock to be used for that queue. As a result, locking can now be
1082 per-queue, with a provision for sharing a lock across queues if
1083 necessary (e.g the scsi layer sets the queue lock pointers to the
1084 corresponding adapter lock, which results in a per host locking
1085 granularity). The locking semantics are the same, i.e. locking is
1086 still imposed by the block layer, grabbing the lock before
1087 request_fn execution which it means that lots of older drivers
1088 should still be SMP safe. Drivers are free to drop the queue
1089 lock themselves, if required. Drivers that explicitly used the
1090 io_request_lock for serialization need to be modified accordingly.
1091 Usually it's as easy as adding a global lock:
1093 static spinlock_t my_driver_lock = SPIN_LOCK_UNLOCKED;
1095 and passing the address to that lock to blk_init_queue().
1097 5.2 64 bit sector numbers (sector_t prepares for 64 bit support)
1099 The sector number used in the bio structure has been changed to sector_t,
1100 which could be defined as 64 bit in preparation for 64 bit sector support.
1102 6. Other Changes/Implications
1104 6.1 Partition re-mapping handled by the generic block layer
1106 In 2.5 some of the gendisk/partition related code has been reorganized.
1107 Now the generic block layer performs partition-remapping early and thus
1108 provides drivers with a sector number relative to whole device, rather than
1109 having to take partition number into account in order to arrive at the true
1110 sector number. The routine blk_partition_remap() is invoked by
1111 generic_make_request even before invoking the queue specific make_request_fn,
1112 so the i/o scheduler also gets to operate on whole disk sector numbers. This
1113 should typically not require changes to block drivers, it just never gets
1114 to invoke its own partition sector offset calculations since all bios
1115 sent are offset from the beginning of the device.
1118 7. A Few Tips on Migration of older drivers
1120 Old-style drivers that just use CURRENT and ignores clustered requests,
1121 may not need much change. The generic layer will automatically handle
1122 clustered requests, multi-page bios, etc for the driver.
1124 For a low performance driver or hardware that is PIO driven or just doesn't
1125 support scatter-gather changes should be minimal too.
1127 The following are some points to keep in mind when converting old drivers
1128 to bio.
1130 Drivers should use elv_next_request to pick up requests and are no longer
1131 supposed to handle looping directly over the request list.
1132 (struct request->queue has been removed)
1134 Now end_that_request_first takes an additional number_of_sectors argument.
1135 It used to handle always just the first buffer_head in a request, now
1136 it will loop and handle as many sectors (on a bio-segment granularity)
1137 as specified.
1139 Now bh->b_end_io is replaced by bio->bi_end_io, but most of the time the
1140 right thing to use is bio_endio(bio, uptodate) instead.
1142 If the driver is dropping the io_request_lock from its request_fn strategy,
1143 then it just needs to replace that with q->queue_lock instead.
1145 As described in Sec 1.1, drivers can set max sector size, max segment size
1146 etc per queue now. Drivers that used to define their own merge functions i
1147 to handle things like this can now just use the blk_queue_* functions at
1148 blk_init_queue time.
1150 Drivers no longer have to map a {partition, sector offset} into the
1151 correct absolute location anymore, this is done by the block layer, so
1152 where a driver received a request ala this before:
1154 rq->rq_dev = mk_kdev(3, 5); /* /dev/hda5 */
1155 rq->sector = 0; /* first sector on hda5 */
1157 it will now see
1159 rq->rq_dev = mk_kdev(3, 0); /* /dev/hda */
1160 rq->sector = 123128; /* offset from start of disk */
1162 As mentioned, there is no virtual mapping of a bio. For DMA, this is
1163 not a problem as the driver probably never will need a virtual mapping.
1164 Instead it needs a bus mapping (pci_map_page for a single segment or
1165 use blk_rq_map_sg for scatter gather) to be able to ship it to the driver. For
1166 PIO drivers (or drivers that need to revert to PIO transfer once in a
1167 while (IDE for example)), where the CPU is doing the actual data
1168 transfer a virtual mapping is needed. If the driver supports highmem I/O,
1169 (Sec 1.1, (ii) ) it needs to use __bio_kmap_atomic and bio_kmap_irq to
1170 temporarily map a bio into the virtual address space.
1173 8. Prior/Related/Impacted patches
1175 8.1. Earlier kiobuf patches (sct/axboe/chait/hch/mkp)
1176 - orig kiobuf & raw i/o patches (now in 2.4 tree)
1177 - direct kiobuf based i/o to devices (no intermediate bh's)
1178 - page i/o using kiobuf
1179 - kiobuf splitting for lvm (mkp)
1180 - elevator support for kiobuf request merging (axboe)
1181 8.2. Zero-copy networking (Dave Miller)
1182 8.3. SGI XFS - pagebuf patches - use of kiobufs
1183 8.4. Multi-page pioent patch for bio (Christoph Hellwig)
1184 8.5. Direct i/o implementation (Andrea Arcangeli) since 2.4.10-pre11
1185 8.6. Async i/o implementation patch (Ben LaHaise)
1186 8.7. EVMS layering design (IBM EVMS team)
1187 8.8. Larger page cache size patch (Ben LaHaise) and
1188 Large page size (Daniel Phillips)
1189 => larger contiguous physical memory buffers
1190 8.9. VM reservations patch (Ben LaHaise)
1191 8.10. Write clustering patches ? (Marcelo/Quintela/Riel ?)
1192 8.11. Block device in page cache patch (Andrea Archangeli) - now in 2.4.10+
1193 8.12. Multiple block-size transfers for faster raw i/o (Shailabh Nagar,
1194 Badari)
1195 8.13 Priority based i/o scheduler - prepatches (Arjan van de Ven)
1196 8.14 IDE Taskfile i/o patch (Andre Hedrick)
1197 8.15 Multi-page writeout and readahead patches (Andrew Morton)
1198 8.16 Direct i/o patches for 2.5 using kvec and bio (Badari Pulavarthy)
1200 9. Other References:
1202 9.1 The Splice I/O Model - Larry McVoy (and subsequent discussions on lkml,
1203 and Linus' comments - Jan 2001)
1204 9.2 Discussions about kiobuf and bh design on lkml between sct, linus, alan
1205 et al - Feb-March 2001 (many of the initial thoughts that led to bio were
1206 brought up in this discusion thread)
1207 9.3 Discussions on mempool on lkml - Dec 2001.