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Diffstat (limited to 'Documentation/filesystems/porting.rst')
| -rw-r--r-- | Documentation/filesystems/porting.rst | 246 |
1 files changed, 229 insertions, 17 deletions
diff --git a/Documentation/filesystems/porting.rst b/Documentation/filesystems/porting.rst index aee9aaf9f3df..1be76ef117b3 100644 --- a/Documentation/filesystems/porting.rst +++ b/Documentation/filesystems/porting.rst @@ -177,7 +177,7 @@ settles down a bit. **mandatory** s_export_op is now required for exporting a filesystem. -isofs, ext2, ext3, resierfs, fat +isofs, ext2, ext3, reiserfs, fat can be used as examples of very different filesystems. --- @@ -462,15 +462,15 @@ ERR_PTR(...). 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_acl -to read an ACL from disk. +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 hanle this by returning -EINVAL, but it would be nicer to +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. @@ -517,7 +517,7 @@ The witch is dead! Well, 2/3 of it, anyway. ->d_revalidate() and ->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 tha argument - they are guaranteed that the +local filesystems can ignore this argument - they are guaranteed that the object doesn't exist. It's remote/distributed ones that might care... --- @@ -537,7 +537,7 @@ vfs_readdir() is gone; switch to iterate_dir() instead **mandatory** -->readdir() is gone now; switch to ->iterate() +->readdir() is gone now; switch to ->iterate_shared() **mandatory** @@ -693,24 +693,19 @@ parallel now. --- -**recommended** +**mandatory** -->iterate_shared() is added; it's a parallel variant of ->iterate(). +->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. -Often enough ->iterate() can serve as ->iterate_shared() without any -changes - it is a read-only operation, after all. If you have any -per-inode or per-dentry in-core data structures modified by ->iterate(), -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. - -Old method is only used if the new one is absent; eventually it will -be removed. Switch while you still can; the old one won't stay. +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. --- @@ -922,3 +917,220 @@ is provided - file_open_root_mnt(). In-tree users adjusted. 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 severly +underdocumented however: + +(1) 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. + +(2) 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. + +(3) 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: + +(1) 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. + +(2) 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). + +(3) 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. |