view Documentation/filesystems/porting @ 524:7f8b544237bf

netfront: Allow netfront in domain 0.

This is useful if your physical network device is in a utility domain.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
author Keir Fraser <keir.fraser@citrix.com>
date Tue Apr 15 15:18:58 2008 +0100 (2008-04-15)
parents 831230e53067
line source
1 Changes since 2.5.0:
3 ---
4 [recommended]
6 New helpers: sb_bread(), sb_getblk(), sb_find_get_block(), set_bh(),
7 sb_set_blocksize() and sb_min_blocksize().
9 Use them.
11 (sb_find_get_block() replaces 2.4's get_hash_table())
13 ---
14 [recommended]
16 New methods: ->alloc_inode() and ->destroy_inode().
18 Remove inode->u.foo_inode_i
19 Declare
20 struct foo_inode_info {
21 /* fs-private stuff */
22 struct inode vfs_inode;
23 };
24 static inline struct foo_inode_info *FOO_I(struct inode *inode)
25 {
26 return list_entry(inode, struct foo_inode_info, vfs_inode);
27 }
29 Use FOO_I(inode) instead of &inode->u.foo_inode_i;
31 Add foo_alloc_inode() and foo_destory_inode() - the former should allocate
32 foo_inode_info and return the address of ->vfs_inode, the latter should free
33 FOO_I(inode) (see in-tree filesystems for examples).
35 Make them ->alloc_inode and ->destroy_inode in your super_operations.
37 Keep in mind that now you need explicit initialization of private data -
38 typically in ->read_inode() and after getting an inode from new_inode().
40 At some point that will become mandatory.
42 ---
43 [mandatory]
45 Change of file_system_type method (->read_super to ->get_sb)
47 ->read_super() is no more. Ditto for DECLARE_FSTYPE and DECLARE_FSTYPE_DEV.
49 Turn your foo_read_super() into a function that would return 0 in case of
50 success and negative number in case of error (-EINVAL unless you have more
51 informative error value to report). Call it foo_fill_super(). Now declare
53 int foo_get_sb(struct file_system_type *fs_type,
54 int flags, const char *dev_name, void *data, struct vfsmount *mnt)
55 {
56 return get_sb_bdev(fs_type, flags, dev_name, data, foo_fill_super,
57 mnt);
58 }
60 (or similar with s/bdev/nodev/ or s/bdev/single/, depending on the kind of
61 filesystem).
63 Replace DECLARE_FSTYPE... with explicit initializer and have ->get_sb set as
64 foo_get_sb.
66 ---
67 [mandatory]
69 Locking change: ->s_vfs_rename_sem is taken only by cross-directory renames.
70 Most likely there is no need to change anything, but if you relied on
71 global exclusion between renames for some internal purpose - you need to
72 change your internal locking. Otherwise exclusion warranties remain the
73 same (i.e. parents and victim are locked, etc.).
75 ---
76 [informational]
78 Now we have the exclusion between ->lookup() and directory removal (by
79 ->rmdir() and ->rename()). If you used to need that exclusion and do
80 it by internal locking (most of filesystems couldn't care less) - you
81 can relax your locking.
83 ---
84 [mandatory]
86 ->lookup(), ->truncate(), ->create(), ->unlink(), ->mknod(), ->mkdir(),
87 ->rmdir(), ->link(), ->lseek(), ->symlink(), ->rename()
88 and ->readdir() are called without BKL now. Grab it on entry, drop upon return
89 - that will guarantee the same locking you used to have. If your method or its
90 parts do not need BKL - better yet, now you can shift lock_kernel() and
91 unlock_kernel() so that they would protect exactly what needs to be
92 protected.
94 ---
95 [mandatory]
97 BKL is also moved from around sb operations. ->write_super() Is now called
98 without BKL held. BKL should have been shifted into individual fs sb_op
99 functions. If you don't need it, remove it.
101 ---
102 [informational]
104 check for ->link() target not being a directory is done by callers. Feel
105 free to drop it...
107 ---
108 [informational]
110 ->link() callers hold ->i_sem on the object we are linking to. Some of your
111 problems might be over...
113 ---
114 [mandatory]
116 new file_system_type method - kill_sb(superblock). If you are converting
117 an existing filesystem, set it according to ->fs_flags:
118 FS_REQUIRES_DEV - kill_block_super
119 FS_LITTER - kill_litter_super
120 neither - kill_anon_super
121 FS_LITTER is gone - just remove it from fs_flags.
123 ---
124 [mandatory]
126 FS_SINGLE is gone (actually, that had happened back when ->get_sb()
127 went in - and hadn't been documented ;-/). Just remove it from fs_flags
128 (and see ->get_sb() entry for other actions).
130 ---
131 [mandatory]
133 ->setattr() is called without BKL now. Caller _always_ holds ->i_sem, so
134 watch for ->i_sem-grabbing code that might be used by your ->setattr().
135 Callers of notify_change() need ->i_sem now.
137 ---
138 [recommended]
140 New super_block field "struct export_operations *s_export_op" for
141 explicit support for exporting, e.g. via NFS. The structure is fully
142 documented at its declaration in include/linux/fs.h, and in
143 Documentation/filesystems/Exporting.
145 Briefly it allows for the definition of decode_fh and encode_fh operations
146 to encode and decode filehandles, and allows the filesystem to use
147 a standard helper function for decode_fh, and provide file-system specific
148 support for this helper, particularly get_parent.
150 It is planned that this will be required for exporting once the code
151 settles down a bit.
153 [mandatory]
155 s_export_op is now required for exporting a filesystem.
156 isofs, ext2, ext3, resierfs, fat
157 can be used as examples of very different filesystems.
159 ---
160 [mandatory]
162 iget4() and the read_inode2 callback have been superseded by iget5_locked()
163 which has the following prototype,
165 struct inode *iget5_locked(struct super_block *sb, unsigned long ino,
166 int (*test)(struct inode *, void *),
167 int (*set)(struct inode *, void *),
168 void *data);
170 'test' is an additional function that can be used when the inode
171 number is not sufficient to identify the actual file object. 'set'
172 should be a non-blocking function that initializes those parts of a
173 newly created inode to allow the test function to succeed. 'data' is
174 passed as an opaque value to both test and set functions.
176 When the inode has been created by iget5_locked(), it will be returned with
177 the I_NEW flag set and will still be locked. read_inode has not been
178 called so the file system still has to finalize the initialization. Once
179 the inode is initialized it must be unlocked by calling unlock_new_inode().
181 The filesystem is responsible for setting (and possibly testing) i_ino
182 when appropriate. There is also a simpler iget_locked function that
183 just takes the superblock and inode number as arguments and does the
184 test and set for you.
186 e.g.
187 inode = iget_locked(sb, ino);
188 if (inode->i_state & I_NEW) {
189 read_inode_from_disk(inode);
190 unlock_new_inode(inode);
191 }
193 ---
194 [recommended]
196 ->getattr() finally getting used. See instances in nfs, minix, etc.
198 ---
199 [mandatory]
201 ->revalidate() is gone. If your filesystem had it - provide ->getattr()
202 and let it call whatever you had as ->revlidate() + (for symlinks that
203 had ->revalidate()) add calls in ->follow_link()/->readlink().
205 ---
206 [mandatory]
208 ->d_parent changes are not protected by BKL anymore. Read access is safe
209 if at least one of the following is true:
210 * filesystem has no cross-directory rename()
211 * dcache_lock is held
212 * we know that parent had been locked (e.g. we are looking at
213 ->d_parent of ->lookup() argument).
214 * we are called from ->rename().
215 * the child's ->d_lock is held
216 Audit your code and add locking if needed. Notice that any place that is
217 not protected by the conditions above is risky even in the old tree - you
218 had been relying on BKL and that's prone to screwups. Old tree had quite
219 a few holes of that kind - unprotected access to ->d_parent leading to
220 anything from oops to silent memory corruption.
222 ---
223 [mandatory]
225 FS_NOMOUNT is gone. If you use it - just set MS_NOUSER in flags
226 (see rootfs for one kind of solution and bdev/socket/pipe for another).
228 ---
229 [recommended]
231 Use bdev_read_only(bdev) instead of is_read_only(kdev). The latter
232 is still alive, but only because of the mess in drivers/s390/block/dasd.c.
233 As soon as it gets fixed is_read_only() will die.
235 ---
236 [mandatory]
238 ->permission() is called without BKL now. Grab it on entry, drop upon
239 return - that will guarantee the same locking you used to have. If
240 your method or its parts do not need BKL - better yet, now you can
241 shift lock_kernel() and unlock_kernel() so that they would protect
242 exactly what needs to be protected.
244 ---
245 [mandatory]
247 ->statfs() is now called without BKL held. BKL should have been
248 shifted into individual fs sb_op functions where it's not clear that
249 it's safe to remove it. If you don't need it, remove it.
251 ---
252 [mandatory]
254 is_read_only() is gone; use bdev_read_only() instead.
256 ---
257 [mandatory]
259 destroy_buffers() is gone; use invalidate_bdev().
261 ---
262 [mandatory]
264 fsync_dev() is gone; use fsync_bdev(). NOTE: lvm breakage is
265 deliberate; as soon as struct block_device * is propagated in a reasonable
266 way by that code fixing will become trivial; until then nothing can be
267 done.