Changes since 2.5.0:¶
---
recommended
New helpers: sb_bread(), sb_getblk(), sb_find_get_block(), set_bh(), sb_set_blocksize() and sb_min_blocksize().
Use them.
(sb_find_get_block() replaces 2.4’s get_hash_table())
---
recommended
New methods: ->alloc_inode() and ->destroy_inode().
Remove inode->u.foo_inode_i
Declare:
struct foo_inode_info {
/* fs-private stuff */
struct inode vfs_inode;
};
static inline struct foo_inode_info *FOO_I(struct inode *inode)
{
return list_entry(inode, struct foo_inode_info, vfs_inode);
}
Use FOO_I(inode) instead of &inode->u.foo_inode_i;
Add foo_alloc_inode() and foo_destroy_inode() - the former should allocate foo_inode_info and return the address of ->vfs_inode, the latter should free FOO_I(inode) (see in-tree filesystems for examples).
Make them ->alloc_inode and ->destroy_inode in your super_operations.
Keep in mind that now you need explicit initialization of private data
typically between calling iget_locked() and unlocking the inode.
At some point that will become mandatory.
mandatory
The foo_inode_info should always be allocated through alloc_inode_sb() rather
than kmem_cache_alloc() or kmalloc() related to set up the inode reclaim context
correctly.
---
mandatory
Change of file_system_type method (->read_super to ->get_sb)
->read_super() is no more. Ditto for DECLARE_FSTYPE and DECLARE_FSTYPE_DEV.
Turn your foo_read_super() into a function that would return 0 in case of success and negative number in case of error (-EINVAL unless you have more informative error value to report). Call it foo_fill_super(). Now declare:
int foo_get_sb(struct file_system_type *fs_type,
int flags, const char *dev_name, void *data, struct vfsmount *mnt)
{
return get_sb_bdev(fs_type, flags, dev_name, data, foo_fill_super,
mnt);
}
(or similar with s/bdev/nodev/ or s/bdev/single/, depending on the kind of filesystem).
Replace DECLARE_FSTYPE... with explicit initializer and have ->get_sb set as foo_get_sb.
---
mandatory
Locking change: ->s_vfs_rename_sem is taken only by cross-directory renames. Most likely there is no need to change anything, but if you relied on global exclusion between renames for some internal purpose - you need to change your internal locking. Otherwise exclusion warranties remain the same (i.e. parents and victim are locked, etc.).
---
informational
Now we have the exclusion between ->lookup() and directory removal (by ->rmdir() and ->rename()). If you used to need that exclusion and do it by internal locking (most of filesystems couldn’t care less) - you can relax your locking.
---
mandatory
->lookup(), ->truncate(), ->create(), ->unlink(), ->mknod(), ->mkdir(), ->rmdir(), ->link(), ->lseek(), ->symlink(), ->rename() and ->readdir() are called without BKL now. Grab it on entry, drop upon return - that will guarantee the same locking you used to have. If your method or its parts do not need BKL - better yet, now you can shift lock_kernel() and unlock_kernel() so that they would protect exactly what needs to be protected.
---
mandatory
BKL is also moved from around sb operations. BKL should have been shifted into individual fs sb_op functions. If you don’t need it, remove it.
---
informational
check for ->link() target not being a directory is done by callers. Feel free to drop it...
---
informational
->link() callers hold ->i_mutex on the object we are linking to. Some of your problems might be over...
---
mandatory
new file_system_type method - kill_sb(superblock). If you are converting an existing filesystem, set it according to ->fs_flags:
FS_REQUIRES_DEV - kill_block_super
FS_LITTER - kill_litter_super
neither - kill_anon_super
FS_LITTER is gone - just remove it from fs_flags.
---
mandatory
FS_SINGLE is gone (actually, that had happened back when ->get_sb() went in - and hadn’t been documented ;-/). Just remove it from fs_flags (and see ->get_sb() entry for other actions).
---
mandatory
->setattr() is called without BKL now. Caller _always_ holds ->i_mutex, so
watch for ->i_mutex-grabbing code that might be used by your ->setattr().
Callers of notify_change() need ->i_mutex now.
---
recommended
New super_block field struct export_operations *s_export_op for
explicit support for exporting, e.g. via NFS. The structure is fully
documented at its declaration in include/linux/fs.h, and in
Making Filesystems Exportable.
Briefly it allows for the definition of decode_fh and encode_fh operations to encode and decode filehandles, and allows the filesystem to use a standard helper function for decode_fh, and provide file-system specific support for this helper, particularly get_parent.
It is planned that this will be required for exporting once the code settles down a bit.
mandatory
s_export_op is now required for exporting a filesystem. isofs, ext2, ext3, fat can be used as examples of very different filesystems.
---
mandatory
iget4() and the read_inode2 callback have been superseded by iget5_locked()
which has the following prototype:
struct inode *iget5_locked(struct super_block *sb, unsigned long ino,
int (*test)(struct inode *, void *),
int (*set)(struct inode *, void *),
void *data);
‘test’ is an additional function that can be used when the inode number is not sufficient to identify the actual file object. ‘set’ should be a non-blocking function that initializes those parts of a newly created inode to allow the test function to succeed. ‘data’ is passed as an opaque value to both test and set functions.
When the inode has been created by iget5_locked(), it will be returned with the
I_NEW flag set and will still be locked. The filesystem then needs to finalize
the initialization. Once the inode is initialized it must be unlocked by
calling unlock_new_inode().
The filesystem is responsible for setting (and possibly testing) i_ino when appropriate. There is also a simpler iget_locked function that just takes the superblock and inode number as arguments and does the test and set for you.
e.g.:
inode = iget_locked(sb, ino);
if (inode->i_state & I_NEW) {
err = read_inode_from_disk(inode);
if (err < 0) {
iget_failed(inode);
return err;
}
unlock_new_inode(inode);
}
Note that if the process of setting up a new inode fails, then iget_failed()
should be called on the inode to render it dead, and an appropriate error
should be passed back to the caller.
---
recommended
->getattr() finally getting used. See instances in nfs, minix, etc.
---
mandatory
->revalidate() is gone. If your filesystem had it - provide ->getattr() and let it call whatever you had as ->revlidate() + (for symlinks that had ->revalidate()) add calls in ->follow_link()/->readlink().
---
mandatory
->d_parent changes are not protected by BKL anymore. Read access is safe if at least one of the following is true:
filesystem has no cross-directory rename()
we know that parent had been locked (e.g. we are looking at ->d_parent of ->lookup() argument).
we are called from ->rename().
the child’s ->d_lock is held
Audit your code and add locking if needed. Notice that any place that is not protected by the conditions above is risky even in the old tree - you had been relying on BKL and that’s prone to screwups. Old tree had quite a few holes of that kind - unprotected access to ->d_parent leading to anything from oops to silent memory corruption.
---
mandatory
FS_NOMOUNT is gone. If you use it - just set SB_NOUSER in flags (see rootfs for one kind of solution and bdev/socket/pipe for another).
---
recommended
Use bdev_read_only(bdev) instead of is_read_only(kdev). The latter is still alive, but only because of the mess in drivers/s390/block/dasd.c. As soon as it gets fixed is_read_only() will die.
---
mandatory
->permission() is called without BKL now. Grab it on entry, drop upon return - that will guarantee the same locking you used to have. If your method or its parts do not need BKL - better yet, now you can shift lock_kernel() and unlock_kernel() so that they would protect exactly what needs to be protected.
---
mandatory
->statfs() is now called without BKL held. BKL should have been shifted into individual fs sb_op functions where it’s not clear that it’s safe to remove it. If you don’t need it, remove it.
---
mandatory
is_read_only() is gone; use bdev_read_only() instead.
---
mandatory
destroy_buffers() is gone; use invalidate_bdev().
---
mandatory
fsync_dev() is gone; use fsync_bdev(). NOTE: lvm breakage is deliberate; as soon as struct block_device * is propagated in a reasonable way by that code fixing will become trivial; until then nothing can be done.
mandatory
block truncation on error exit from ->write_begin, and ->direct_IO moved from generic methods (block_write_begin, cont_write_begin, nobh_write_begin, blockdev_direct_IO*) to callers. Take a look at ext2_write_failed and callers for an example.
mandatory
->truncate is gone. The whole truncate sequence needs to be implemented in ->setattr, which is now mandatory for filesystems implementing on-disk size changes. Start with a copy of the old inode_setattr and vmtruncate, and the reorder the vmtruncate + foofs_vmtruncate sequence to be in order of zeroing blocks using block_truncate_page or similar helpers, size update and on finally on-disk truncation which should not fail. setattr_prepare (which used to be inode_change_ok) now includes the size checks for ATTR_SIZE and must be called in the beginning of ->setattr unconditionally.
mandatory
->clear_inode() and ->delete_inode() are gone; ->evict_inode() should
be used instead. It gets called whenever the inode is evicted, whether it has
remaining links or not. Caller does not evict the pagecache or inode-associated
metadata buffers; the method has to use truncate_inode_pages_final() to get rid
of those. Caller makes sure async writeback cannot be running for the inode while
(or after) ->evict_inode() is called.
->drop_inode() returns int now; it’s called on final iput() with
inode->i_lock held and it returns true if filesystems wants the inode to be
dropped. As before, generic_drop_inode() is still the default and it’s been
updated appropriately. generic_delete_inode() is also alive and it consists
simply of return 1. Note that all actual eviction work is done by caller after
->drop_inode() returns.
As before, clear_inode() must be called exactly once on each call of ->evict_inode() (as it used to be for each call of ->delete_inode()). Unlike before, if you are using inode-associated metadata buffers (i.e. mark_buffer_dirty_inode()), it’s your responsibility to call invalidate_inode_buffers() before clear_inode().
NOTE: checking i_nlink in the beginning of ->write_inode() and bailing out
if it’s zero is not and never had been enough. Final unlink() and iput()
may happen while the inode is in the middle of ->write_inode(); e.g. if you blindly
free the on-disk inode, you may end up doing that while ->write_inode() is writing
to it.
---
mandatory
.d_delete() now only advises the dcache as to whether or not to cache
unreferenced dentries, and is now only called when the dentry refcount goes to
0. Even on 0 refcount transition, it must be able to tolerate being called 0,
1, or more times (eg. constant, idempotent).
---
mandatory
.d_compare() calling convention and locking rules are significantly changed. Read updated documentation in Overview of the Linux Virtual File System (and look at examples of other filesystems) for guidance.
---
mandatory
.d_hash() calling convention and locking rules are significantly changed. Read updated documentation in Overview of the Linux Virtual File System (and look at examples of other filesystems) for guidance.
---
mandatory
dcache_lock is gone, replaced by fine grained locks. See fs/dcache.c for details of what locks to replace dcache_lock with in order to protect particular things. Most of the time, a filesystem only needs ->d_lock, which protects all the dcache state of a given dentry.
---
mandatory
Filesystems must RCU-free their inodes, if they can have been accessed via rcu-walk path walk (basically, if the file can have had a path name in the vfs namespace).
Even though i_dentry and i_rcu share storage in a union, we will initialize the former in inode_init_always(), so just leave it alone in the callback. It used to be necessary to clean it there, but not anymore (starting at 3.2).
---
recommended
vfs now tries to do path walking in “rcu-walk mode”, which avoids atomic operations and scalability hazards on dentries and inodes (see Pathname lookup). d_hash and d_compare changes (above) are examples of the changes required to support this. For more complex filesystem callbacks, the vfs drops out of rcu-walk mode before the fs call, so no changes are required to the filesystem. However, this is costly and loses the benefits of rcu-walk mode. We will begin to add filesystem callbacks that are rcu-walk aware, shown below. Filesystems should take advantage of this where possible.
---
mandatory
d_revalidate is a callback that is made on every path element (if the filesystem provides it), which requires dropping out of rcu-walk mode. This may now be called in rcu-walk mode (nd->flags & LOOKUP_RCU). -ECHILD should be returned if the filesystem cannot handle rcu-walk. See Overview of the Linux Virtual File System for more details.
permission is an inode permission check that is called on many or all directory inodes on the way down a path walk (to check for exec permission). It must now be rcu-walk aware (mask & MAY_NOT_BLOCK). See Overview of the Linux Virtual File System for more details.
---
mandatory
In ->fallocate() you must check the mode option passed in. If your filesystem does not support hole punching (deallocating space in the middle of a file) you must return -EOPNOTSUPP if FALLOC_FL_PUNCH_HOLE is set in mode. Currently you can only have FALLOC_FL_PUNCH_HOLE with FALLOC_FL_KEEP_SIZE set, so the i_size should not change when hole punching, even when puching the end of a file off.
---
mandatory
->get_sb() is gone. Switch to use of ->mount(). Typically it’s just
a matter of switching from calling get_sb_... to mount_... and changing
the function type. If you were doing it manually, just switch from setting
->mnt_root to some pointer to returning that pointer. On errors return
ERR_PTR(...).
---
mandatory
->permission() and generic_permission()have lost flags
argument; instead of passing IPERM_FLAG_RCU we add MAY_NOT_BLOCK into mask.
generic_permission() has also lost the check_acl argument; ACL checking
has been taken to VFS and filesystems need to provide a non-NULL
->i_op->get_inode_acl to read an ACL from disk.
---
mandatory
If you implement your own ->llseek() you must handle SEEK_HOLE and SEEK_DATA. You can handle this by returning -EINVAL, but it would be nicer to support it in some way. The generic handler assumes that the entire file is data and there is a virtual hole at the end of the file. So if the provided offset is less than i_size and SEEK_DATA is specified, return the same offset. If the above is true for the offset and you are given SEEK_HOLE, return the end of the file. If the offset is i_size or greater return -ENXIO in either case.
mandatory
If you have your own ->fsync() you must make sure to call
filemap_write_and_wait_range() so that all dirty pages are synced out properly.
You must also keep in mind that ->fsync() is not called with i_mutex held
anymore, so if you require i_mutex locking you must make sure to take it and
release it yourself.
---
mandatory
d_alloc_root() is gone, along with a lot of bugs caused by code misusing it. Replacement: d_make_root(inode). On success d_make_root(inode) allocates and returns a new dentry instantiated with the passed in inode. On failure NULL is returned and the passed in inode is dropped so the reference to inode is consumed in all cases and failure handling need not do any cleanup for the inode. If d_make_root(inode) is passed a NULL inode it returns NULL and also requires no further error handling. Typical usage is:
inode = foofs_new_inode(....);
s->s_root = d_make_root(inode);
if (!s->s_root)
/* Nothing needed for the inode cleanup */
return -ENOMEM;
...
---
mandatory
The witch is dead! Well, 2/3 of it, anyway. ->d_revalidate() and ->lookup() do not take struct nameidata anymore; just the flags.
---
mandatory
->create() doesn’t take struct nameidata *; unlike the previous
two, it gets “is it an O_EXCL or equivalent?” boolean argument. Note that
local filesystems can ignore this argument - they are guaranteed that the
object doesn’t exist. It’s remote/distributed ones that might care...
---
mandatory
FS_REVAL_DOT is gone; if you used to have it, add ->d_weak_revalidate() in your dentry operations instead.
---
mandatory
vfs_readdir() is gone; switch to iterate_dir() instead
---
mandatory
->readdir() is gone now; switch to ->iterate_shared()
mandatory
vfs_follow_link has been removed. Filesystems must use nd_set_link from ->follow_link for normal symlinks, or nd_jump_link for magic /proc/<pid> style links.
---
mandatory
iget5_locked()/ilookup5()/ilookup5_nowait() test() callback used to be
called with both ->i_lock and inode_hash_lock held; the former is not
taken anymore, so verify that your callbacks do not rely on it (none
of the in-tree instances did). inode_hash_lock is still held,
of course, so they are still serialized wrt removal from inode hash,
as well as wrt set() callback of iget5_locked().
---
mandatory
d_materialise_unique() is gone; d_splice_alias() does everything you
need now. Remember that they have opposite orders of arguments ;-/
---
mandatory
f_dentry is gone; use f_path.dentry, or, better yet, see if you can avoid it entirely.
---
mandatory
never call ->read() and ->write() directly; use __vfs_{read,write} or wrappers; instead of checking for ->write or ->read being NULL, look for FMODE_CAN_{WRITE,READ} in file->f_mode.
---
mandatory
do _not_ use new_sync_{read,write} for ->read/->write; leave it NULL instead.
---
- mandatory
->aio_read/->aio_write are gone. Use ->read_iter/->write_iter.
---
recommended
for embedded (“fast”) symlinks just set inode->i_link to wherever the symlink body is and use simple_follow_link() as ->follow_link().
---
mandatory
calling conventions for ->follow_link() have changed. Instead of returning cookie and using nd_set_link() to store the body to traverse, we return the body to traverse and store the cookie using explicit void ** argument. nameidata isn’t passed at all - nd_jump_link() doesn’t need it and nd_[gs]et_link() is gone.
---
mandatory
calling conventions for ->put_link() have changed. It gets inode instead of dentry, it does not get nameidata at all and it gets called only when cookie is non-NULL. Note that link body isn’t available anymore, so if you need it, store it as cookie.
---
mandatory
any symlink that might use page_follow_link_light/page_put_link() must have inode_nohighmem(inode) called before anything might start playing with its pagecache. No highmem pages should end up in the pagecache of such symlinks. That includes any preseeding that might be done during symlink creation. page_symlink() will honour the mapping gfp flags, so once you’ve done inode_nohighmem() it’s safe to use, but if you allocate and insert the page manually, make sure to use the right gfp flags.
---
mandatory
->follow_link() is replaced with ->get_link(); same API, except that
->get_link() gets inode as a separate argument
->get_link() may be called in RCU mode - in that case NULL dentry is passed
---
mandatory
->get_link() gets struct delayed_call *done now, and should do
set_delayed_call() where it used to set *cookie.
->put_link() is gone - just give the destructor to set_delayed_call() in ->get_link().
---
mandatory
->getxattr() and xattr_handler.get() get dentry and inode passed separately.
dentry might be yet to be attached to inode, so do _not_ use its ->d_inode
in the instances. Rationale: !@#!@# security_d_instantiate() needs to be
called before we attach dentry to inode.
---
mandatory
symlinks are no longer the only inodes that do not have i_bdev/i_cdev/ i_pipe/i_link union zeroed out at inode eviction. As the result, you can’t assume that non-NULL value in ->i_nlink at ->destroy_inode() implies that it’s a symlink. Checking ->i_mode is really needed now. In-tree we had to fix shmem_destroy_callback() that used to take that kind of shortcut; watch out, since that shortcut is no longer valid.
---
mandatory
->i_mutex is replaced with ->i_rwsem now. inode_lock() et.al. work as they used to - they just take it exclusive. However, ->lookup() may be called with parent locked shared. Its instances must not
use d_instantiate) and
d_rehash()separately - used_add()ord_splice_alias()instead.use
d_rehash()alone - call d_add(new_dentry, NULL) instead.in the unlikely case when (read-only) access to filesystem data structures needs exclusion for some reason, arrange it yourself. None of the in-tree filesystems needed that.
rely on ->d_parent and ->d_name not changing after dentry has been fed to
d_add()ord_splice_alias(). Again, none of the in-tree instances relied upon that.
We are guaranteed that lookups of the same name in the same directory will not happen in parallel (“same” in the sense of your ->d_compare()). Lookups on different names in the same directory can and do happen in parallel now.
---
mandatory
->iterate_shared() is added.
Exclusion on struct file level is still provided (as well as that
between it and lseek on the same struct file), but if your directory
has been opened several times, you can get these called in parallel.
Exclusion between that method and all directory-modifying ones is
still provided, of course.
If you have any per-inode or per-dentry in-core data structures modified by ->iterate_shared(), you might need something to serialize the access to them. If you do dcache pre-seeding, you’ll need to switch to d_alloc_parallel() for that; look for in-tree examples.
---
mandatory
->atomic_open() calls without O_CREAT may happen in parallel.
---
mandatory
->setxattr() and xattr_handler.set() get dentry and inode passed separately.
The xattr_handler.set() gets passed the user namespace of the mount the inode
is seen from so filesystems can idmap the i_uid and i_gid accordingly.
dentry might be yet to be attached to inode, so do _not_ use its ->d_inode
in the instances. Rationale: !@#!@# security_d_instantiate() needs to be
called before we attach dentry to inode and !@#!@##!@$!$#!@#$!@$!@$ smack
->d_instantiate() uses not just ->getxattr() but ->setxattr() as well.
---
mandatory
->d_compare() doesn’t get parent as a separate argument anymore. If you used it for finding the struct super_block involved, dentry->d_sb will work just as well; if it’s something more complicated, use dentry->d_parent. Just be careful not to assume that fetching it more than once will yield the same value - in RCU mode it could change under you.
---
mandatory
->rename() has an added flags argument. Any flags not handled by the filesystem should result in EINVAL being returned.
---
recommended
->readlink is optional for symlinks. Don’t set, unless filesystem needs to fake something for readlink(2).
---
mandatory
->getattr() is now passed a struct path rather than a vfsmount and dentry separately, and it now has request_mask and query_flags arguments to specify the fields and sync type requested by statx. Filesystems not supporting any statx-specific features may ignore the new arguments.
---
mandatory
->atomic_open() calling conventions have changed. Gone is int *opened,
along with FILE_OPENED/FILE_CREATED. In place of those we have
FMODE_OPENED/FMODE_CREATED, set in file->f_mode. Additionally, return
value for ‘called finish_no_open(), open it yourself’ case has become
0, not 1. Since finish_no_open() itself is returning 0 now, that part
does not need any changes in ->atomic_open() instances.
---
mandatory
alloc_file() has become static now; two wrappers are to be used instead.
alloc_file_pseudo(inode, vfsmount, name, flags, ops) is for the cases
when dentry needs to be created; that’s the majority of old alloc_file()
users. Calling conventions: on success a reference to new struct file
is returned and callers reference to inode is subsumed by that. On
failure, ERR_PTR() is returned and no caller’s references are affected,
so the caller needs to drop the inode reference it held.
alloc_file_clone(file, flags, ops) does not affect any caller’s references.
On success you get a new struct file sharing the mount/dentry with the
original, on failure - ERR_PTR().
---
mandatory
->clone_file_range() and ->dedupe_file_range have been replaced with ->remap_file_range(). See Overview of the Linux Virtual File System for more information.
---
recommended
->lookup() instances doing an equivalent of:
if (IS_ERR(inode))
return ERR_CAST(inode);
return d_splice_alias(inode, dentry);
don’t need to bother with the check - d_splice_alias() will do the
right thing when given ERR_PTR(...) as inode. Moreover, passing NULL
inode to d_splice_alias() will also do the right thing (equivalent of
d_add(dentry, NULL); return NULL;), so that kind of special cases
also doesn’t need a separate treatment.
---
strongly recommended
take the RCU-delayed parts of ->destroy_inode() into a new method - ->free_inode(). If ->destroy_inode() becomes empty - all the better, just get rid of it. Synchronous work (e.g. the stuff that can’t be done from an RCU callback, or any WARN_ON() where we want the stack trace) might be movable to ->evict_inode(); however, that goes only for the things that are not needed to balance something done by ->alloc_inode(). IOW, if it’s cleaning up the stuff that might have accumulated over the life of in-core inode, ->evict_inode() might be a fit.
Rules for inode destruction:
if ->destroy_inode() is non-NULL, it gets called
if ->free_inode() is non-NULL, it gets scheduled by
call_rcu()combination of NULL ->destroy_inode and NULL ->free_inode is treated as NULL/free_inode_nonrcu, to preserve the compatibility.
Note that the callback (be it via ->free_inode() or explicit call_rcu()
in ->destroy_inode()) is NOT ordered wrt superblock destruction;
as the matter of fact, the superblock and all associated structures
might be already gone. The filesystem driver is guaranteed to be still
there, but that’s it. Freeing memory in the callback is fine; doing
more than that is possible, but requires a lot of care and is best
avoided.
---
mandatory
DCACHE_RCUACCESS is gone; having an RCU delay on dentry freeing is the default. DCACHE_NORCU opts out, and only d_alloc_pseudo() has any business doing so.
---
mandatory
d_alloc_pseudo() is internal-only; uses outside of alloc_file_pseudo() are very suspect (and won’t work in modules). Such uses are very likely to be misspelled d_alloc_anon().
---
mandatory
[should’ve been added in 2016] stale comment in finish_open() notwithstanding, failure exits in ->atomic_open() instances should NOT fput() the file, no matter what. Everything is handled by the caller.
---
mandatory
clone_private_mount() returns a longterm mount now, so the proper destructor of
its result is kern_unmount() or kern_unmount_array().
---
mandatory
zero-length bvec segments are disallowed, they must be filtered out before passed on to an iterator.
---
mandatory
For bvec based itererators bio_iov_iter_get_pages() now doesn’t copy bvecs but
uses the one provided. Anyone issuing kiocb-I/O should ensure that the bvec and
page references stay until I/O has completed, i.e. until ->ki_complete() has
been called or returned with non -EIOCBQUEUED code.
---
mandatory
mnt_want_write_file() can now only be paired with mnt_drop_write_file(),
whereas previously it could be paired with mnt_drop_write() as well.
---
mandatory
iov_iter_copy_from_user_atomic() is gone; use copy_page_from_iter_atomic(). The difference is copy_page_from_iter_atomic() advances the iterator and you don’t need iov_iter_advance() after it. However, if you decide to use only a part of obtained data, you should do iov_iter_revert().
---
mandatory
Calling conventions for file_open_root() changed; now it takes struct path * instead of passing mount and dentry separately. For callers that used to pass <mnt, mnt->mnt_root> pair (i.e. the root of given mount), a new helper is provided - file_open_root_mnt(). In-tree users adjusted.
---
mandatory
no_llseek is gone; don’t set .llseek to that - just leave it NULL instead. Checks for “does that file have llseek(2), or should it fail with ESPIPE” should be done by looking at FMODE_LSEEK in file->f_mode.
---
mandatory
filldir_t (readdir callbacks) calling conventions have changed. Instead of returning 0 or -E... it returns bool now. false means “no more” (as -E... used to) and true - “keep going” (as 0 in old calling conventions). Rationale: callers never looked at specific -E... values anyway. -> iterate_shared() instances require no changes at all, all filldir_t ones in the tree converted.
---
mandatory
Calling conventions for ->tmpfile() have changed. It now takes a struct
file pointer instead of struct dentry pointer. d_tmpfile() is similarly
changed to simplify callers. The passed file is in a non-open state and on
success must be opened before returning (e.g. by calling
finish_open_simple()).
---
mandatory
Calling convention for ->huge_fault has changed. It now takes a page order instead of an enum page_entry_size, and it may be called without the mmap_lock held. All in-tree users have been audited and do not seem to depend on the mmap_lock being held, but out of tree users should verify for themselves. If they do need it, they can return VM_FAULT_RETRY to be called with the mmap_lock held.
---
mandatory
The order of opening block devices and matching or creating superblocks has changed.
The old logic opened block devices first and then tried to find a suitable superblock to reuse based on the block device pointer.
The new logic tries to find a suitable superblock first based on the device number, and opening the block device afterwards.
Since opening block devices cannot happen under s_umount because of lock ordering requirements s_umount is now dropped while opening block devices and reacquired before calling fill_super().
In the old logic concurrent mounters would find the superblock on the list of superblocks for the filesystem type. Since the first opener of the block device would hold s_umount they would wait until the superblock became either born or was discarded due to initialization failure.
Since the new logic drops s_umount concurrent mounters could grab s_umount and would spin. Instead they are now made to wait using an explicit wait-wake mechanism without having to hold s_umount.
---
mandatory
The holder of a block device is now the superblock.
The holder of a block device used to be the file_system_type which wasn’t particularly useful. It wasn’t possible to go from block device to owning superblock without matching on the device pointer stored in the superblock. This mechanism would only work for a single device so the block layer couldn’t find the owning superblock of any additional devices.
In the old mechanism reusing or creating a superblock for a racing mount(2) and umount(2) relied on the file_system_type as the holder. This was severely underdocumented however:
Any concurrent mounter that managed to grab an active reference on an existing superblock was made to wait until the superblock either became ready or until the superblock was removed from the list of superblocks of the filesystem type. If the superblock is ready the caller would simple reuse it.
If the mounter came after
deactivate_locked_super()but before the superblock had been removed from the list of superblocks of the filesystem type the mounter would wait until the superblock was shutdown, reuse the block device and allocate a new superblock.If the mounter came after
deactivate_locked_super()and after the superblock had been removed from the list of superblocks of the filesystem type the mounter would reuse the block device and allocate a new superblock (the bd_holder point may still be set to the filesystem type).
Because the holder of the block device was the file_system_type any concurrent mounter could open the block devices of any superblock of the same file_system_type without risking seeing EBUSY because the block device was still in use by another superblock.
Making the superblock the owner of the block device changes this as the holder is now a unique superblock and thus block devices associated with it cannot be reused by concurrent mounters. So a concurrent mounter in (2) could suddenly see EBUSY when trying to open a block device whose holder was a different superblock.
The new logic thus waits until the superblock and the devices are shutdown in ->kill_sb(). Removal of the superblock from the list of superblocks of the filesystem type is now moved to a later point when the devices are closed:
Any concurrent mounter managing to grab an active reference on an existing superblock is made to wait until the superblock is either ready or until the superblock and all devices are shutdown in ->kill_sb(). If the superblock is ready the caller will simply reuse it.
If the mounter comes after
deactivate_locked_super()but before the superblock has been removed from the list of superblocks of the filesystem type the mounter is made to wait until the superblock and the devices are shut down in ->kill_sb() and the superblock is removed from the list of superblocks of the filesystem type. The mounter will allocate a new superblock and grab ownership of the block device (the bd_holder pointer of the block device will be set to the newly allocated superblock).This case is now collapsed into (2) as the superblock is left on the list of superblocks of the filesystem type until all devices are shutdown in ->kill_sb(). In other words, if the superblock isn’t on the list of superblock of the filesystem type anymore then it has given up ownership of all associated block devices (the bd_holder pointer is NULL).
As this is a VFS level change it has no practical consequences for filesystems other than that all of them must use one of the provided kill_litter_super(), kill_anon_super(), or kill_block_super() helpers.
---
mandatory
Lock ordering has been changed so that s_umount ranks above open_mutex again. All places where s_umount was taken under open_mutex have been fixed up.
---
mandatory
export_operations ->encode_fh() no longer has a default implementation to
encode FILEID_INO32_GEN* file handles.
Filesystems that used the default implementation may use the generic helper
generic_encode_ino32_fh() explicitly.
---
mandatory
If ->rename() update of .. on cross-directory move needs an exclusion with directory modifications, do not lock the subdirectory in question in your ->rename() - it’s done by the caller now [that item should’ve been added in 28eceeda130f “fs: Lock moved directories”].
---
mandatory
On same-directory ->rename() the (tautological) update of .. is not protected by any locks; just don’t do it if the old parent is the same as the new one. We really can’t lock two subdirectories in same-directory rename - not without deadlocks.
---
mandatory
lock_rename() and lock_rename_child() may fail in cross-directory case, if their arguments do not have a common ancestor. In that case ERR_PTR(-EXDEV) is returned, with no locks taken. In-tree users updated; out-of-tree ones would need to do so.
---
mandatory
The list of children anchored in parent dentry got turned into hlist now. Field names got changed (->d_children/->d_sib instead of ->d_subdirs/->d_child for anchor/entries resp.), so any affected places will be immediately caught by compiler.
---
mandatory
->d_delete() instances are now called for dentries with ->d_lock held
and refcount equal to 0. They are not permitted to drop/regain ->d_lock.
None of in-tree instances did anything of that sort. Make sure yours do not...
---
mandatory
->d_prune() instances are now called without ->d_lock held on the parent. ->d_lock on dentry itself is still held; if you need per-parent exclusions (none of the in-tree instances did), use your own spinlock.
->d_iput() and ->d_release() are called with victim dentry still in the list of parent’s children. It is still unhashed, marked killed, etc., just not removed from parent’s ->d_children yet.
Anyone iterating through the list of children needs to be aware of the half-killed dentries that might be seen there; taking ->d_lock on those will see them negative, unhashed and with negative refcount, which means that most of the in-kernel users would’ve done the right thing anyway without any adjustment.
---
recommended
Block device freezing and thawing have been moved to holder operations.
Before this change, get_active_super() would only be able to find the superblock of the main block device, i.e., the one stored in sb->s_bdev. Block device freezing now works for any block device owned by a given superblock, not just the main block device. The get_active_super() helper and bd_fsfreeze_sb pointer are gone.
---
mandatory
set_blocksize() takes opened struct file instead of struct block_device now
and it must be opened exclusive.