Adding upstream version 5.10.209.upstream/5.10.209upstream

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

1 files changed, 666 insertions, 0 deletions

diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst
new file mode 100644
index 000000000..c0f2c7586
--- /dev/null
+++ b/Documentation/filesystems/locking.rst
@@ -0,0 +1,666 @@
+=======
+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);
+	int (*get_acl)(struct inode *, int);
+	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 inode *, struct dentry *, umode_t);
+
+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 (all)	(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
+============	=============================================
+
+
+	Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_rwsem
+	exclusive on victim.
+	cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem.
+
+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 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);
+	int (*bdev_try_to_free_page)(struct super_block*, struct page*, gfp_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)
+bdev_try_to_free_page:	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.
+
+->bdev_try_to_free_page is called from the ->releasepage handler of
+the block device inode.  See there for more details.
+
+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 (*readpage)(struct file *, struct page *);
+	int (*writepages)(struct address_space *, struct writeback_control *);
+	int (*set_page_dirty)(struct page *page);
+	void (*readahead)(struct readahead_control *);
+	int (*readpages)(struct file *filp, struct address_space *mapping,
+			struct list_head *pages, unsigned nr_pages);
+	int (*write_begin)(struct file *, struct address_space *mapping,
+				loff_t pos, unsigned len, unsigned flags,
+				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 (*invalidatepage) (struct page *, unsigned int, unsigned int);
+	int (*releasepage) (struct page *, int);
+	void (*freepage)(struct page *);
+	int (*direct_IO)(struct kiocb *, struct iov_iter *iter);
+	bool (*isolate_page) (struct page *, isolate_mode_t);
+	int (*migratepage)(struct address_space *, struct page *, struct page *);
+	void (*putback_page) (struct page *);
+	int (*launder_page)(struct page *);
+	int (*is_partially_uptodate)(struct page *, unsigned long, unsigned long);
+	int (*error_remove_page)(struct address_space *, struct page *);
+	int (*swap_activate)(struct file *);
+	int (*swap_deactivate)(struct file *);
+
+locking rules:
+	All except set_page_dirty and freepage may block
+
+======================	======================== =========
+ops			PageLocked(page)	 i_rwsem
+======================	======================== =========
+writepage:		yes, unlocks (see below)
+readpage:		yes, unlocks
+writepages:
+set_page_dirty		no
+readahead:		yes, unlocks
+readpages:		no
+write_begin:		locks the page		 exclusive
+write_end:		yes, unlocks		 exclusive
+bmap:
+invalidatepage:		yes
+releasepage:		yes
+freepage:		yes
+direct_IO:
+isolate_page:		yes
+migratepage:		yes (both)
+putback_page:		yes
+launder_page:		yes
+is_partially_uptodate:	yes
+error_remove_page:	yes
+swap_activate:		no
+swap_deactivate:	no
+======================	======================== =========
+
+->write_begin(), ->write_end() and ->readpage() may be called from
+the request handler (/dev/loop).
+
+->readpage() unlocks the page, either synchronously or via I/O
+completion.
+
+->readahead() unlocks the pages that I/O is attempted on like ->readpage().
+
+->readpages() populates the pagecache with the passed pages and starts
+I/O against them.  They come unlocked upon I/O completion.
+
+->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.
+
+->set_page_dirty() is called from various places in the kernel
+when the target page is marked as needing writeback.  It may be called
+under spinlock (it cannot block) and is sometimes called with the page
+not locked.
+
+->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.
+
+->invalidatepage() 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. If ->invalidatepage is zero, the kernel uses
+block_invalidatepage() instead.
+
+->releasepage() is called when the kernel is about to try to drop the
+buffers from the page in preparation for freeing it.  It returns zero to
+indicate that the buffers are (or may be) freeable.  If ->releasepage is zero,
+the kernel assumes that the fs has no private interest in the buffers.
+
+->freepage() is called when the kernel is done dropping the page
+from the page cache.
+
+->launder_page() may be called prior to releasing a page if
+it is still found to be dirty. It returns zero if the page was successfully
+cleaned, or an error value if not. Note that in order to prevent the page
+getting mapped back in and redirtied, it needs to be kept locked
+across the entire operation.
+
+->swap_activate will be called with a non-zero argument on
+files backing (non block device backed) swapfiles. A return value
+of zero indicates success, in which case this file can be used for
+backing swapspace. The swapspace operations will be proxied to the
+address space operations.
+
+->swap_deactivate() will be called in the sys_swapoff()
+path after ->swap_activate() returned success.
+
+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 *);
+
+locking rules:
+
+======================	=============	=================	=========
+ops			inode->i_lock	blocked_lock_lock	may block
+======================	=============	=================	=========
+lm_notify:		yes		yes			no
+lm_grant:		no		no			no
+lm_break:		yes		no			no
+lm_change		yes		no			no
+lm_breaker_owns_lease:	no		no			no
+======================	=============	=================	=========
+
+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 (*revalidate_disk) (struct gendisk *);
+	int (*getgeo)(struct block_device *, struct hd_geometry *);
+	void (*swap_slot_free_notify) (struct block_device *, unsigned long);
+
+locking rules:
+
+======================= ===================
+ops			bd_mutex
+======================= ===================
+open:			yes
+release:		yes
+ioctl:			no
+compat_ioctl:		no
+direct_access:		no
+unlock_native_capacity:	no
+revalidate_disk:	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 (*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 (*readv) (struct file *, const struct iovec *, unsigned long,
+			loff_t *);
+	ssize_t (*writev) (struct file *, const struct iovec *, unsigned long,
+			loff_t *);
+	ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t,
+			void __user *);
+	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);
+
+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
+
+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
+the page, then ensure it is not already truncated (the page 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, and then return with the page locked. 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)