view Documentation/rpc-cache.txt @ 452:c7ed6fe5dca0

kexec: dont initialise regions in reserve_memory()

There is no need to initialise efi_memmap_res and boot_param_res in
reserve_memory() for the initial xen domain as it is done in
machine_kexec_setup_resources() using values from the kexec hypercall.

Signed-off-by: Simon Horman <horms@verge.net.au>
author Keir Fraser <keir.fraser@citrix.com>
date Thu Feb 28 10:55:18 2008 +0000 (2008-02-28)
parents 831230e53067
line source
1 This document gives a brief introduction to the caching
2 mechanisms in the sunrpc layer that is used, in particular,
3 for NFS authentication.
6 ======
7 The caching replaces the old exports table and allows for
8 a wide variety of values to be caches.
10 There are a number of caches that are similar in structure though
11 quite possibly very different in content and use. There is a corpus
12 of common code for managing these caches.
14 Examples of caches that are likely to be needed are:
15 - mapping from IP address to client name
16 - mapping from client name and filesystem to export options
17 - mapping from UID to list of GIDs, to work around NFS's limitation
18 of 16 gids.
19 - mappings between local UID/GID and remote UID/GID for sites that
20 do not have uniform uid assignment
21 - mapping from network identify to public key for crypto authentication.
23 The common code handles such things as:
24 - general cache lookup with correct locking
25 - supporting 'NEGATIVE' as well as positive entries
26 - allowing an EXPIRED time on cache items, and removing
27 items after they expire, and are no longe in-use.
28 - making requests to user-space to fill in cache entries
29 - allowing user-space to directly set entries in the cache
30 - delaying RPC requests that depend on as-yet incomplete
31 cache entries, and replaying those requests when the cache entry
32 is complete.
33 - clean out old entries as they expire.
35 Creating a Cache
36 ----------------
38 1/ A cache needs a datum to store. This is in the form of a
39 structure definition that must contain a
40 struct cache_head
41 as an element, usually the first.
42 It will also contain a key and some content.
43 Each cache element is reference counted and contains
44 expiry and update times for use in cache management.
45 2/ A cache needs a "cache_detail" structure that
46 describes the cache. This stores the hash table, some
47 parameters for cache management, and some operations detailing how
48 to work with particular cache items.
49 The operations requires are:
50 struct cache_head *alloc(void)
51 This simply allocates appropriate memory and returns
52 a pointer to the cache_detail embedded within the
53 structure
54 void cache_put(struct kref *)
55 This is called when the last reference to an item is
56 is dropped. The pointer passed is to the 'ref' field
57 in the cache_head. cache_put should release any
58 references create by 'cache_init' and, if CACHE_VALID
59 is set, any references created by cache_update.
60 It should then release the memory allocated by
61 'alloc'.
62 int match(struct cache_head *orig, struct cache_head *new)
63 test if the keys in the two structures match. Return
64 1 if they do, 0 if they don't.
65 void init(struct cache_head *orig, struct cache_head *new)
66 Set the 'key' fields in 'new' from 'orig'. This may
67 include taking references to shared objects.
68 void update(struct cache_head *orig, struct cache_head *new)
69 Set the 'content' fileds in 'new' from 'orig'.
70 int cache_show(struct seq_file *m, struct cache_detail *cd,
71 struct cache_head *h)
72 Optional. Used to provide a /proc file that lists the
73 contents of a cache. This should show one item,
74 usually on just one line.
75 int cache_request(struct cache_detail *cd, struct cache_head *h,
76 char **bpp, int *blen)
77 Format a request to be send to user-space for an item
78 to be instantiated. *bpp is a buffer of size *blen.
79 bpp should be moved forward over the encoded message,
80 and *blen should be reduced to show how much free
81 space remains. Return 0 on success or <0 if not
82 enough room or other problem.
83 int cache_parse(struct cache_detail *cd, char *buf, int len)
84 A message from user space has arrived to fill out a
85 cache entry. It is in 'buf' of length 'len'.
86 cache_parse should parse this, find the item in the
87 cache with sunrpc_cache_lookup, and update the item
88 with sunrpc_cache_update.
91 3/ A cache needs to be registered using cache_register(). This
92 includes it on a list of caches that will be regularly
93 cleaned to discard old data.
95 Using a cache
96 -------------
98 To find a value in a cache, call sunrpc_cache_lookup passing a pointer
99 to the cache_head in a sample item with the 'key' fields filled in.
100 This will be passed to ->match to identify the target entry. If no
101 entry is found, a new entry will be create, added to the cache, and
102 marked as not containing valid data.
104 The item returned is typically passed to cache_check which will check
105 if the data is valid, and may initiate an up-call to get fresh data.
106 cache_check will return -ENOENT in the entry is negative or if an up
107 call is needed but not possible, -EAGAIN if an upcall is pending,
108 or 0 if the data is valid;
110 cache_check can be passed a "struct cache_req *". This structure is
111 typically embedded in the actual request and can be used to create a
112 deferred copy of the request (struct cache_deferred_req). This is
113 done when the found cache item is not uptodate, but the is reason to
114 believe that userspace might provide information soon. When the cache
115 item does become valid, the deferred copy of the request will be
116 revisited (->revisit). It is expected that this method will
117 reschedule the request for processing.
119 The value returned by sunrpc_cache_lookup can also be passed to
120 sunrpc_cache_update to set the content for the item. A second item is
121 passed which should hold the content. If the item found by _lookup
122 has valid data, then it is discarded and a new item is created. This
123 saves any user of an item from worrying about content changing while
124 it is being inspected. If the item found by _lookup does not contain
125 valid data, then the content is copied across and CACHE_VALID is set.
127 Populating a cache
128 ------------------
130 Each cache has a name, and when the cache is registered, a directory
131 with that name is created in /proc/net/rpc
133 This directory contains a file called 'channel' which is a channel
134 for communicating between kernel and user for populating the cache.
135 This directory may later contain other files of interacting
136 with the cache.
138 The 'channel' works a bit like a datagram socket. Each 'write' is
139 passed as a whole to the cache for parsing and interpretation.
140 Each cache can treat the write requests differently, but it is
141 expected that a message written will contain:
142 - a key
143 - an expiry time
144 - a content.
145 with the intention that an item in the cache with the give key
146 should be create or updated to have the given content, and the
147 expiry time should be set on that item.
149 Reading from a channel is a bit more interesting. When a cache
150 lookup fails, or when it succeeds but finds an entry that may soon
151 expire, a request is lodged for that cache item to be updated by
152 user-space. These requests appear in the channel file.
154 Successive reads will return successive requests.
155 If there are no more requests to return, read will return EOF, but a
156 select or poll for read will block waiting for another request to be
157 added.
159 Thus a user-space helper is likely to:
160 open the channel.
161 select for readable
162 read a request
163 write a response
164 loop.
166 If it dies and needs to be restarted, any requests that have not been
167 answered will still appear in the file and will be read by the new
168 instance of the helper.
170 Each cache should define a "cache_parse" method which takes a message
171 written from user-space and processes it. It should return an error
172 (which propagates back to the write syscall) or 0.
174 Each cache should also define a "cache_request" method which
175 takes a cache item and encodes a request into the buffer
176 provided.
178 Note: If a cache has no active readers on the channel, and has had not
179 active readers for more than 60 seconds, further requests will not be
180 added to the channel but instead all lookups that do not find a valid
181 entry will fail. This is partly for backward compatibility: The
182 previous nfs exports table was deemed to be authoritative and a
183 failed lookup meant a definite 'no'.
185 request/response format
186 -----------------------
188 While each cache is free to use it's own format for requests
189 and responses over channel, the following is recommended as
190 appropriate and support routines are available to help:
191 Each request or response record should be printable ASCII
192 with precisely one newline character which should be at the end.
193 Fields within the record should be separated by spaces, normally one.
194 If spaces, newlines, or nul characters are needed in a field they
195 much be quoted. two mechanisms are available:
196 1/ If a field begins '\x' then it must contain an even number of
197 hex digits, and pairs of these digits provide the bytes in the
198 field.
199 2/ otherwise a \ in the field must be followed by 3 octal digits
200 which give the code for a byte. Other characters are treated
201 as them selves. At the very least, space, newline, nul, and
202 '\' must be quoted in this way.