=======
  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 flags);

  	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)