Adding upstream version 6.1.76.upstream/6.1.76

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 697 insertions, 0 deletions

diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst
new file mode 100644
index 000000000..555215c41
--- /dev/null
+++ b/Documentation/filesystems/locking.rst
@@ -0,0 +1,697 @@
+=======
+Locking
+=======
+
+The text below describes the locking rules for VFS-related methods.
+It is (believed to be) up-to-date. *Please*, if you change anything in
+prototypes or locking protocols - update this file. And update the relevant
+instances in the tree, don't leave that to maintainers of filesystems/devices/
+etc. At the very least, put the list of dubious cases in the end of this file.
+Don't turn it into log - maintainers of out-of-the-tree code are supposed to
+be able to use diff(1).
+
+Thing currently missing here: socket operations. Alexey?
+
+dentry_operations
+=================
+
+prototypes::
+
+	int (*d_revalidate)(struct dentry *, unsigned int);
+	int (*d_weak_revalidate)(struct dentry *, unsigned int);
+	int (*d_hash)(const struct dentry *, struct qstr *);
+	int (*d_compare)(const struct dentry *,
+			unsigned int, const char *, const struct qstr *);
+	int (*d_delete)(struct dentry *);
+	int (*d_init)(struct dentry *);
+	void (*d_release)(struct dentry *);
+	void (*d_iput)(struct dentry *, struct inode *);
+	char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);
+	struct vfsmount *(*d_automount)(struct path *path);
+	int (*d_manage)(const struct path *, bool);
+	struct dentry *(*d_real)(struct dentry *, const struct inode *);
+
+locking rules:
+
+================== ===========	========	==============	========
+ops		   rename_lock	->d_lock	may block	rcu-walk
+================== ===========	========	==============	========
+d_revalidate:	   no		no		yes (ref-walk)	maybe
+d_weak_revalidate: no		no		yes	 	no
+d_hash		   no		no		no		maybe
+d_compare:	   yes		no		no		maybe
+d_delete:	   no		yes		no		no
+d_init:		   no		no		yes		no
+d_release:	   no		no		yes		no
+d_prune:           no		yes		no		no
+d_iput:		   no		no		yes		no
+d_dname:	   no		no		no		no
+d_automount:	   no		no		yes		no
+d_manage:	   no		no		yes (ref-walk)	maybe
+d_real		   no		no		yes 		no
+================== ===========	========	==============	========
+
+inode_operations
+================
+
+prototypes::
+
+	int (*create) (struct inode *,struct dentry *,umode_t, bool);
+	struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
+	int (*link) (struct dentry *,struct inode *,struct dentry *);
+	int (*unlink) (struct inode *,struct dentry *);
+	int (*symlink) (struct inode *,struct dentry *,const char *);
+	int (*mkdir) (struct inode *,struct dentry *,umode_t);
+	int (*rmdir) (struct inode *,struct dentry *);
+	int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t);
+	int (*rename) (struct inode *, struct dentry *,
+			struct inode *, struct dentry *, unsigned int);
+	int (*readlink) (struct dentry *, char __user *,int);
+	const char *(*get_link) (struct dentry *, struct inode *, struct delayed_call *);
+	void (*truncate) (struct inode *);
+	int (*permission) (struct inode *, int, unsigned int);
+	struct posix_acl * (*get_acl)(struct inode *, int, bool);
+	int (*setattr) (struct dentry *, struct iattr *);
+	int (*getattr) (const struct path *, struct kstat *, u32, unsigned int);
+	ssize_t (*listxattr) (struct dentry *, char *, size_t);
+	int (*fiemap)(struct inode *, struct fiemap_extent_info *, u64 start, u64 len);
+	void (*update_time)(struct inode *, struct timespec *, int);
+	int (*atomic_open)(struct inode *, struct dentry *,
+				struct file *, unsigned open_flag,
+				umode_t create_mode);
+	int (*tmpfile) (struct user_namespace *, struct inode *,
+			struct file *, umode_t);
+	int (*fileattr_set)(struct user_namespace *mnt_userns,
+			    struct dentry *dentry, struct fileattr *fa);
+	int (*fileattr_get)(struct dentry *dentry, struct fileattr *fa);
+
+locking rules:
+	all may block
+
+=============	=============================================
+ops		i_rwsem(inode)
+=============	=============================================
+lookup:		shared
+create:		exclusive
+link:		exclusive (both)
+mknod:		exclusive
+symlink:	exclusive
+mkdir:		exclusive
+unlink:		exclusive (both)
+rmdir:		exclusive (both)(see below)
+rename:		exclusive (both parents, some children)	(see below)
+readlink:	no
+get_link:	no
+setattr:	exclusive
+permission:	no (may not block if called in rcu-walk mode)
+get_acl:	no
+getattr:	no
+listxattr:	no
+fiemap:		no
+update_time:	no
+atomic_open:	shared (exclusive if O_CREAT is set in open flags)
+tmpfile:	no
+fileattr_get:	no or exclusive
+fileattr_set:	exclusive
+=============	=============================================
+
+
+	Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_rwsem
+	exclusive on victim.
+	cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem.
+	->unlink() and ->rename() have ->i_rwsem exclusive on all non-directories
+	involved.
+	->rename() has ->i_rwsem exclusive on any subdirectory that changes parent.
+
+See Documentation/filesystems/directory-locking.rst for more detailed discussion
+of the locking scheme for directory operations.
+
+xattr_handler operations
+========================
+
+prototypes::
+
+	bool (*list)(struct dentry *dentry);
+	int (*get)(const struct xattr_handler *handler, struct dentry *dentry,
+		   struct inode *inode, const char *name, void *buffer,
+		   size_t size);
+	int (*set)(const struct xattr_handler *handler,
+                   struct user_namespace *mnt_userns,
+                   struct dentry *dentry, struct inode *inode, const char *name,
+                   const void *buffer, size_t size, int flags);
+
+locking rules:
+	all may block
+
+=====		==============
+ops		i_rwsem(inode)
+=====		==============
+list:		no
+get:		no
+set:		exclusive
+=====		==============
+
+super_operations
+================
+
+prototypes::
+
+	struct inode *(*alloc_inode)(struct super_block *sb);
+	void (*free_inode)(struct inode *);
+	void (*destroy_inode)(struct inode *);
+	void (*dirty_inode) (struct inode *, int flags);
+	int (*write_inode) (struct inode *, struct writeback_control *wbc);
+	int (*drop_inode) (struct inode *);
+	void (*evict_inode) (struct inode *);
+	void (*put_super) (struct super_block *);
+	int (*sync_fs)(struct super_block *sb, int wait);
+	int (*freeze_fs) (struct super_block *);
+	int (*unfreeze_fs) (struct super_block *);
+	int (*statfs) (struct dentry *, struct kstatfs *);
+	int (*remount_fs) (struct super_block *, int *, char *);
+	void (*umount_begin) (struct super_block *);
+	int (*show_options)(struct seq_file *, struct dentry *);
+	ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
+	ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
+
+locking rules:
+	All may block [not true, see below]
+
+======================	============	========================
+ops			s_umount	note
+======================	============	========================
+alloc_inode:
+free_inode:				called from RCU callback
+destroy_inode:
+dirty_inode:
+write_inode:
+drop_inode:				!!!inode->i_lock!!!
+evict_inode:
+put_super:		write
+sync_fs:		read
+freeze_fs:		write
+unfreeze_fs:		write
+statfs:			maybe(read)	(see below)
+remount_fs:		write
+umount_begin:		no
+show_options:		no		(namespace_sem)
+quota_read:		no		(see below)
+quota_write:		no		(see below)
+======================	============	========================
+
+->statfs() has s_umount (shared) when called by ustat(2) (native or
+compat), but that's an accident of bad API; s_umount is used to pin
+the superblock down when we only have dev_t given us by userland to
+identify the superblock.  Everything else (statfs(), fstatfs(), etc.)
+doesn't hold it when calling ->statfs() - superblock is pinned down
+by resolving the pathname passed to syscall.
+
+->quota_read() and ->quota_write() functions are both guaranteed to
+be the only ones operating on the quota file by the quota code (via
+dqio_sem) (unless an admin really wants to screw up something and
+writes to quota files with quotas on). For other details about locking
+see also dquot_operations section.
+
+file_system_type
+================
+
+prototypes::
+
+	struct dentry *(*mount) (struct file_system_type *, int,
+		       const char *, void *);
+	void (*kill_sb) (struct super_block *);
+
+locking rules:
+
+=======		=========
+ops		may block
+=======		=========
+mount		yes
+kill_sb		yes
+=======		=========
+
+->mount() returns ERR_PTR or the root dentry; its superblock should be locked
+on return.
+
+->kill_sb() takes a write-locked superblock, does all shutdown work on it,
+unlocks and drops the reference.
+
+address_space_operations
+========================
+prototypes::
+
+	int (*writepage)(struct page *page, struct writeback_control *wbc);
+	int (*read_folio)(struct file *, struct folio *);
+	int (*writepages)(struct address_space *, struct writeback_control *);
+	bool (*dirty_folio)(struct address_space *, struct folio *folio);
+	void (*readahead)(struct readahead_control *);
+	int (*write_begin)(struct file *, struct address_space *mapping,
+				loff_t pos, unsigned len,
+				struct page **pagep, void **fsdata);
+	int (*write_end)(struct file *, struct address_space *mapping,
+				loff_t pos, unsigned len, unsigned copied,
+				struct page *page, void *fsdata);
+	sector_t (*bmap)(struct address_space *, sector_t);
+	void (*invalidate_folio) (struct folio *, size_t start, size_t len);
+	bool (*release_folio)(struct folio *, gfp_t);
+	void (*free_folio)(struct folio *);
+	int (*direct_IO)(struct kiocb *, struct iov_iter *iter);
+	int (*migrate_folio)(struct address_space *, struct folio *dst,
+			struct folio *src, enum migrate_mode);
+	int (*launder_folio)(struct folio *);
+	bool (*is_partially_uptodate)(struct folio *, size_t from, size_t count);
+	int (*error_remove_page)(struct address_space *, struct page *);
+	int (*swap_activate)(struct swap_info_struct *sis, struct file *f, sector_t *span)
+	int (*swap_deactivate)(struct file *);
+	int (*swap_rw)(struct kiocb *iocb, struct iov_iter *iter);
+
+locking rules:
+	All except dirty_folio and free_folio may block
+
+======================	======================== =========	===============
+ops			folio locked		 i_rwsem	invalidate_lock
+======================	======================== =========	===============
+writepage:		yes, unlocks (see below)
+read_folio:		yes, unlocks				shared
+writepages:
+dirty_folio:		maybe
+readahead:		yes, unlocks				shared
+write_begin:		locks the page		 exclusive
+write_end:		yes, unlocks		 exclusive
+bmap:
+invalidate_folio:	yes					exclusive
+release_folio:		yes
+free_folio:		yes
+direct_IO:
+migrate_folio:		yes (both)
+launder_folio:		yes
+is_partially_uptodate:	yes
+error_remove_page:	yes
+swap_activate:		no
+swap_deactivate:	no
+swap_rw:		yes, unlocks
+======================	======================== =========	===============
+
+->write_begin(), ->write_end() and ->read_folio() may be called from
+the request handler (/dev/loop).
+
+->read_folio() unlocks the folio, either synchronously or via I/O
+completion.
+
+->readahead() unlocks the folios that I/O is attempted on like ->read_folio().
+
+->writepage() is used for two purposes: for "memory cleansing" and for
+"sync".  These are quite different operations and the behaviour may differ
+depending upon the mode.
+
+If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then
+it *must* start I/O against the page, even if that would involve
+blocking on in-progress I/O.
+
+If writepage is called for memory cleansing (sync_mode ==
+WBC_SYNC_NONE) then its role is to get as much writeout underway as
+possible.  So writepage should try to avoid blocking against
+currently-in-progress I/O.
+
+If the filesystem is not called for "sync" and it determines that it
+would need to block against in-progress I/O to be able to start new I/O
+against the page the filesystem should redirty the page with
+redirty_page_for_writepage(), then unlock the page and return zero.
+This may also be done to avoid internal deadlocks, but rarely.
+
+If the filesystem is called for sync then it must wait on any
+in-progress I/O and then start new I/O.
+
+The filesystem should unlock the page synchronously, before returning to the
+caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE
+value. WRITEPAGE_ACTIVATE means that page cannot really be written out
+currently, and VM should stop calling ->writepage() on this page for some
+time. VM does this by moving page to the head of the active list, hence the
+name.
+
+Unless the filesystem is going to redirty_page_for_writepage(), unlock the page
+and return zero, writepage *must* run set_page_writeback() against the page,
+followed by unlocking it.  Once set_page_writeback() has been run against the
+page, write I/O can be submitted and the write I/O completion handler must run
+end_page_writeback() once the I/O is complete.  If no I/O is submitted, the
+filesystem must run end_page_writeback() against the page before returning from
+writepage.
+
+That is: after 2.5.12, pages which are under writeout are *not* locked.  Note,
+if the filesystem needs the page to be locked during writeout, that is ok, too,
+the page is allowed to be unlocked at any point in time between the calls to
+set_page_writeback() and end_page_writeback().
+
+Note, failure to run either redirty_page_for_writepage() or the combination of
+set_page_writeback()/end_page_writeback() on a page submitted to writepage
+will leave the page itself marked clean but it will be tagged as dirty in the
+radix tree.  This incoherency can lead to all sorts of hard-to-debug problems
+in the filesystem like having dirty inodes at umount and losing written data.
+
+->writepages() is used for periodic writeback and for syscall-initiated
+sync operations.  The address_space should start I/O against at least
+``*nr_to_write`` pages.  ``*nr_to_write`` must be decremented for each page
+which is written.  The address_space implementation may write more (or less)
+pages than ``*nr_to_write`` asks for, but it should try to be reasonably close.
+If nr_to_write is NULL, all dirty pages must be written.
+
+writepages should _only_ write pages which are present on
+mapping->io_pages.
+
+->dirty_folio() is called from various places in the kernel when
+the target folio is marked as needing writeback.  The folio cannot be
+truncated because either the caller holds the folio lock, or the caller
+has found the folio while holding the page table lock which will block
+truncation.
+
+->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some
+filesystems and by the swapper. The latter will eventually go away.  Please,
+keep it that way and don't breed new callers.
+
+->invalidate_folio() is called when the filesystem must attempt to drop
+some or all of the buffers from the page when it is being truncated. It
+returns zero on success.  The filesystem must exclusively acquire
+invalidate_lock before invalidating page cache in truncate / hole punch
+path (and thus calling into ->invalidate_folio) to block races between page
+cache invalidation and page cache filling functions (fault, read, ...).
+
+->release_folio() is called when the kernel is about to try to drop the
+buffers from the folio in preparation for freeing it.  It returns false to
+indicate that the buffers are (or may be) freeable.  If ->release_folio is
+NULL, the kernel assumes that the fs has no private interest in the buffers.
+
+->free_folio() is called when the kernel has dropped the folio
+from the page cache.
+
+->launder_folio() may be called prior to releasing a folio if
+it is still found to be dirty. It returns zero if the folio was successfully
+cleaned, or an error value if not. Note that in order to prevent the folio
+getting mapped back in and redirtied, it needs to be kept locked
+across the entire operation.
+
+->swap_activate() will be called to prepare the given file for swap.  It
+should perform any validation and preparation necessary to ensure that
+writes can be performed with minimal memory allocation.  It should call
+add_swap_extent(), or the helper iomap_swapfile_activate(), and return
+the number of extents added.  If IO should be submitted through
+->swap_rw(), it should set SWP_FS_OPS, otherwise IO will be submitted
+directly to the block device ``sis->bdev``.
+
+->swap_deactivate() will be called in the sys_swapoff()
+path after ->swap_activate() returned success.
+
+->swap_rw will be called for swap IO if SWP_FS_OPS was set by ->swap_activate().
+
+file_lock_operations
+====================
+
+prototypes::
+
+	void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
+	void (*fl_release_private)(struct file_lock *);
+
+
+locking rules:
+
+===================	=============	=========
+ops			inode->i_lock	may block
+===================	=============	=========
+fl_copy_lock:		yes		no
+fl_release_private:	maybe		maybe[1]_
+===================	=============	=========
+
+.. [1]:
+   ->fl_release_private for flock or POSIX locks is currently allowed
+   to block. Leases however can still be freed while the i_lock is held and
+   so fl_release_private called on a lease should not block.
+
+lock_manager_operations
+=======================
+
+prototypes::
+
+	void (*lm_notify)(struct file_lock *);  /* unblock callback */
+	int (*lm_grant)(struct file_lock *, struct file_lock *, int);
+	void (*lm_break)(struct file_lock *); /* break_lease callback */
+	int (*lm_change)(struct file_lock **, int);
+	bool (*lm_breaker_owns_lease)(struct file_lock *);
+        bool (*lm_lock_expirable)(struct file_lock *);
+        void (*lm_expire_lock)(void);
+
+locking rules:
+
+======================	=============	=================	=========
+ops			   flc_lock  	blocked_lock_lock	may block
+======================	=============	=================	=========
+lm_notify:		no      	yes			no
+lm_grant:		no		no			no
+lm_break:		yes		no			no
+lm_change		yes		no			no
+lm_breaker_owns_lease:	yes     	no			no
+lm_lock_expirable	yes		no			no
+lm_expire_lock		no		no			yes
+======================	=============	=================	=========
+
+buffer_head
+===========
+
+prototypes::
+
+	void (*b_end_io)(struct buffer_head *bh, int uptodate);
+
+locking rules:
+
+called from interrupts. In other words, extreme care is needed here.
+bh is locked, but that's all warranties we have here. Currently only RAID1,
+highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices
+call this method upon the IO completion.
+
+block_device_operations
+=======================
+prototypes::
+
+	int (*open) (struct block_device *, fmode_t);
+	int (*release) (struct gendisk *, fmode_t);
+	int (*ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
+	int (*compat_ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
+	int (*direct_access) (struct block_device *, sector_t, void **,
+				unsigned long *);
+	void (*unlock_native_capacity) (struct gendisk *);
+	int (*getgeo)(struct block_device *, struct hd_geometry *);
+	void (*swap_slot_free_notify) (struct block_device *, unsigned long);
+
+locking rules:
+
+======================= ===================
+ops			open_mutex
+======================= ===================
+open:			yes
+release:		yes
+ioctl:			no
+compat_ioctl:		no
+direct_access:		no
+unlock_native_capacity:	no
+getgeo:			no
+swap_slot_free_notify:	no	(see below)
+======================= ===================
+
+swap_slot_free_notify is called with swap_lock and sometimes the page lock
+held.
+
+
+file_operations
+===============
+
+prototypes::
+
+	loff_t (*llseek) (struct file *, loff_t, int);
+	ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
+	ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
+	ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
+	ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
+	int (*iopoll) (struct kiocb *kiocb, bool spin);
+	int (*iterate) (struct file *, struct dir_context *);
+	int (*iterate_shared) (struct file *, struct dir_context *);
+	__poll_t (*poll) (struct file *, struct poll_table_struct *);
+	long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
+	long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
+	int (*mmap) (struct file *, struct vm_area_struct *);
+	int (*open) (struct inode *, struct file *);
+	int (*flush) (struct file *);
+	int (*release) (struct inode *, struct file *);
+	int (*fsync) (struct file *, loff_t start, loff_t end, int datasync);
+	int (*fasync) (int, struct file *, int);
+	int (*lock) (struct file *, int, struct file_lock *);
+	ssize_t (*sendpage) (struct file *, struct page *, int, size_t,
+			loff_t *, int);
+	unsigned long (*get_unmapped_area)(struct file *, unsigned long,
+			unsigned long, unsigned long, unsigned long);
+	int (*check_flags)(int);
+	int (*flock) (struct file *, int, struct file_lock *);
+	ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *,
+			size_t, unsigned int);
+	ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *,
+			size_t, unsigned int);
+	int (*setlease)(struct file *, long, struct file_lock **, void **);
+	long (*fallocate)(struct file *, int, loff_t, loff_t);
+	void (*show_fdinfo)(struct seq_file *m, struct file *f);
+	unsigned (*mmap_capabilities)(struct file *);
+	ssize_t (*copy_file_range)(struct file *, loff_t, struct file *,
+			loff_t, size_t, unsigned int);
+	loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in,
+			struct file *file_out, loff_t pos_out,
+			loff_t len, unsigned int remap_flags);
+	int (*fadvise)(struct file *, loff_t, loff_t, int);
+
+locking rules:
+	All may block.
+
+->llseek() locking has moved from llseek to the individual llseek
+implementations.  If your fs is not using generic_file_llseek, you
+need to acquire and release the appropriate locks in your ->llseek().
+For many filesystems, it is probably safe to acquire the inode
+mutex or just to use i_size_read() instead.
+Note: this does not protect the file->f_pos against concurrent modifications
+since this is something the userspace has to take care about.
+
+->iterate() is called with i_rwsem exclusive.
+
+->iterate_shared() is called with i_rwsem at least shared.
+
+->fasync() is responsible for maintaining the FASYNC bit in filp->f_flags.
+Most instances call fasync_helper(), which does that maintenance, so it's
+not normally something one needs to worry about.  Return values > 0 will be
+mapped to zero in the VFS layer.
+
+->readdir() and ->ioctl() on directories must be changed. Ideally we would
+move ->readdir() to inode_operations and use a separate method for directory
+->ioctl() or kill the latter completely. One of the problems is that for
+anything that resembles union-mount we won't have a struct file for all
+components. And there are other reasons why the current interface is a mess...
+
+->read on directories probably must go away - we should just enforce -EISDIR
+in sys_read() and friends.
+
+->setlease operations should call generic_setlease() before or after setting
+the lease within the individual filesystem to record the result of the
+operation
+
+->fallocate implementation must be really careful to maintain page cache
+consistency when punching holes or performing other operations that invalidate
+page cache contents. Usually the filesystem needs to call
+truncate_inode_pages_range() to invalidate relevant range of the page cache.
+However the filesystem usually also needs to update its internal (and on disk)
+view of file offset -> disk block mapping. Until this update is finished, the
+filesystem needs to block page faults and reads from reloading now-stale page
+cache contents from the disk. Since VFS acquires mapping->invalidate_lock in
+shared mode when loading pages from disk (filemap_fault(), filemap_read(),
+readahead paths), the fallocate implementation must take the invalidate_lock to
+prevent reloading.
+
+->copy_file_range and ->remap_file_range implementations need to serialize
+against modifications of file data while the operation is running. For
+blocking changes through write(2) and similar operations inode->i_rwsem can be
+used. To block changes to file contents via a memory mapping during the
+operation, the filesystem must take mapping->invalidate_lock to coordinate
+with ->page_mkwrite.
+
+dquot_operations
+================
+
+prototypes::
+
+	int (*write_dquot) (struct dquot *);
+	int (*acquire_dquot) (struct dquot *);
+	int (*release_dquot) (struct dquot *);
+	int (*mark_dirty) (struct dquot *);
+	int (*write_info) (struct super_block *, int);
+
+These operations are intended to be more or less wrapping functions that ensure
+a proper locking wrt the filesystem and call the generic quota operations.
+
+What filesystem should expect from the generic quota functions:
+
+==============	============	=========================
+ops		FS recursion	Held locks when called
+==============	============	=========================
+write_dquot:	yes		dqonoff_sem or dqptr_sem
+acquire_dquot:	yes		dqonoff_sem or dqptr_sem
+release_dquot:	yes		dqonoff_sem or dqptr_sem
+mark_dirty:	no		-
+write_info:	yes		dqonoff_sem
+==============	============	=========================
+
+FS recursion means calling ->quota_read() and ->quota_write() from superblock
+operations.
+
+More details about quota locking can be found in fs/dquot.c.
+
+vm_operations_struct
+====================
+
+prototypes::
+
+	void (*open)(struct vm_area_struct*);
+	void (*close)(struct vm_area_struct*);
+	vm_fault_t (*fault)(struct vm_area_struct*, struct vm_fault *);
+	vm_fault_t (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *);
+	vm_fault_t (*pfn_mkwrite)(struct vm_area_struct *, struct vm_fault *);
+	int (*access)(struct vm_area_struct *, unsigned long, void*, int, int);
+
+locking rules:
+
+=============	=========	===========================
+ops		mmap_lock	PageLocked(page)
+=============	=========	===========================
+open:		yes
+close:		yes
+fault:		yes		can return with page locked
+map_pages:	yes
+page_mkwrite:	yes		can return with page locked
+pfn_mkwrite:	yes
+access:		yes
+=============	=========	===========================
+
+->fault() is called when a previously not present pte is about to be faulted
+in. The filesystem must find and return the page associated with the passed in
+"pgoff" in the vm_fault structure. If it is possible that the page may be
+truncated and/or invalidated, then the filesystem must lock invalidate_lock,
+then ensure the page is not already truncated (invalidate_lock will block
+subsequent truncate), and then return with VM_FAULT_LOCKED, and the page
+locked. The VM will unlock the page.
+
+->map_pages() is called when VM asks to map easy accessible pages.
+Filesystem should find and map pages associated with offsets from "start_pgoff"
+till "end_pgoff". ->map_pages() is called with page table locked and must
+not block.  If it's not possible to reach a page without blocking,
+filesystem should skip it. Filesystem should use do_set_pte() to setup
+page table entry. Pointer to entry associated with the page is passed in
+"pte" field in vm_fault structure. Pointers to entries for other offsets
+should be calculated relative to "pte".
+
+->page_mkwrite() is called when a previously read-only pte is about to become
+writeable. The filesystem again must ensure that there are no
+truncate/invalidate races or races with operations such as ->remap_file_range
+or ->copy_file_range, and then return with the page locked. Usually
+mapping->invalidate_lock is suitable for proper serialization. If the page has
+been truncated, the filesystem should not look up a new page like the ->fault()
+handler, but simply return with VM_FAULT_NOPAGE, which will cause the VM to
+retry the fault.
+
+->pfn_mkwrite() is the same as page_mkwrite but when the pte is
+VM_PFNMAP or VM_MIXEDMAP with a page-less entry. Expected return is
+VM_FAULT_NOPAGE. Or one of the VM_FAULT_ERROR types. The default behavior
+after this call is to make the pte read-write, unless pfn_mkwrite returns
+an error.
+
+->access() is called when get_user_pages() fails in
+access_process_vm(), typically used to debug a process through
+/proc/pid/mem or ptrace.  This function is needed only for
+VM_IO | VM_PFNMAP VMAs.
+
+--------------------------------------------------------------------------------
+
+			Dubious stuff
+
+(if you break something or notice that it is broken and do not fix it yourself
+- at least put it here)