Documentation/filesystems/Locking - linux - Git at Google


            
            	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 *, const struct inode *,

            
            			struct qstr *);

            
            	int (*d_compare)(const struct dentry *, const struct inode *,

            
            			const struct dentry *, const struct inode *,

            
            			unsigned int, const char *, const struct qstr *);

            
            	int (*d_delete)(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)(struct dentry *, bool);


            locking rules:

            
            		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_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


            --------------------------- 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 *);

            
            	int (*readlink) (struct dentry *, char __user *,int);

            
            	void * (*follow_link) (struct dentry *, struct nameidata *);

            
            	void (*put_link) (struct dentry *, struct nameidata *, void *);

            
            	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) (struct vfsmount *, struct dentry *, struct kstat *);

            
            	int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);

            
            	ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);

            
            	ssize_t (*listxattr) (struct dentry *, char *, size_t);

            
            	int (*removexattr) (struct dentry *, const char *);

            
            	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 *opened);


            locking rules:

            
            	all may block

            
            		i_mutex(inode)

            
            lookup:		yes

            
            create:		yes

            
            link:		yes (both)

            
            mknod:		yes

            
            symlink:	yes

            
            mkdir:		yes

            
            unlink:		yes (both)

            
            rmdir:		yes (both)	(see below)

            
            rename:		yes (all)	(see below)

            
            readlink:	no

            
            follow_link:	no

            
            put_link:	no

            
            setattr:	yes

            
            permission:	no (may not block if called in rcu-walk mode)

            
            get_acl:	no

            
            getattr:	no

            
            setxattr:	yes

            
            getxattr:	no

            
            listxattr:	no

            
            removexattr:	yes

            
            fiemap:		no

            
            update_time:	no

            
            atomic_open:	yes


            	Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_mutex on

            
            victim.

            
            	cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem.


            See Documentation/filesystems/directory-locking for more detailed discussion

            
            of the locking scheme for directory operations.


            --------------------------- super_operations ---------------------------

            
            prototypes:

            
            	struct inode *(*alloc_inode)(struct super_block *sb);

            
            	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]

            
            			s_umount

            
            alloc_inode:

            
            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:

            
            	int (*get_sb) (struct file_system_type *, int,

            
            		       const char *, void *, struct vfsmount *);

            
            	struct dentry *(*mount) (struct file_system_type *, int,

            
            		       const char *, void *);

            
            	void (*kill_sb) (struct super_block *);

            
            locking rules:

            
            		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 (*sync_page)(struct page *);

            
            	int (*writepages)(struct address_space *, struct writeback_control *);

            
            	int (*set_page_dirty)(struct page *page);

            
            	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);

            
            	int (*invalidatepage) (struct page *, unsigned long);

            
            	int (*releasepage) (struct page *, int);

            
            	void (*freepage)(struct page *);

            
            	int (*direct_IO)(int, struct kiocb *, const struct iovec *iov,

            
            			loff_t offset, unsigned long nr_segs);

            
            	int (*get_xip_mem)(struct address_space *, pgoff_t, int, void **,

            
            				unsigned long *);

            
            	int (*migratepage)(struct address_space *, struct page *, struct page *);

            
            	int (*launder_page)(struct page *);

            
            	int (*is_partially_uptodate)(struct page *, read_descriptor_t *, 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


            			PageLocked(page)	i_mutex

            
            writepage:		yes, unlocks (see below)

            
            readpage:		yes, unlocks

            
            sync_page:		maybe

            
            writepages:

            
            set_page_dirty		no

            
            readpages:

            
            write_begin:		locks the page		yes

            
            write_end:		yes, unlocks		yes

            
            bmap:

            
            invalidatepage:		yes

            
            releasepage:		yes

            
            freepage:		yes

            
            direct_IO:

            
            get_xip_mem:					maybe

            
            migratepage:		yes (both)

            
            launder_page:		yes

            
            is_partially_uptodate:	yes

            
            error_remove_page:	yes

            
            swap_activate:		no

            
            swap_deactivate:	no


            	->write_begin(), ->write_end(), ->sync_page() and ->readpage()

            
            may be called from the request handler (/dev/loop).


            	->readpage() unlocks the page, either synchronously or via I/O

            
            completion.


            	->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.


            	->sync_page() locking rules are not well-defined - usually it is called

            
            with lock on page, but that is not guaranteed. Considering the currently

            
            existing instances of this method ->sync_page() itself doesn't look

            
            well-defined...


            	->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:

            
            			file_lock_lock	may block

            
            fl_copy_lock:		yes		no

            
            fl_release_private:	maybe		no


            ----------------------- lock_manager_operations ---------------------------

            
            prototypes:

            
            	int (*lm_compare_owner)(struct file_lock *, struct file_lock *);

            
            	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);


            locking rules:

            
            			file_lock_lock	may block

            
            lm_compare_owner:	yes		no

            
            lm_notify:		yes		no

            
            lm_grant:		no		no

            
            lm_break:		yes		no

            
            lm_change		yes		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 *);

            
            	int (*media_changed) (struct gendisk *);

            
            	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:

            
            			bd_mutex

            
            open:			yes

            
            release:		yes

            
            ioctl:			no

            
            compat_ioctl:		no

            
            direct_access:		no

            
            media_changed:		no

            
            unlock_native_capacity:	no

            
            revalidate_disk:	no

            
            getgeo:			no

            
            swap_slot_free_notify:	no	(see below)


            media_changed, unlock_native_capacity and revalidate_disk are called only from

            
            check_disk_change().


            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 (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);

            
            	ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);

            
            	int (*readdir) (struct file *, void *, filldir_t);

            
            	unsigned int (*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 (*aio_fsync) (struct kiocb *, 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 **);

            
            	long (*fallocate)(struct file *, int, loff_t, loff_t);

            
            };


            locking rules:

            
            	All may block except for ->setlease.

            
            	No VFS locks held on entry except for ->setlease.


            ->setlease has the file_list_lock held and must not sleep.


            ->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.


            ->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.


            --------------------------- 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:


            		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*);

            
            	int (*fault)(struct vm_area_struct*, struct vm_fault *);

            
            	int (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *);

            
            	int (*access)(struct vm_area_struct *, unsigned long, void*, int, int);


            locking rules:

            
            		mmap_sem	PageLocked(page)

            
            open:		yes

            
            close:		yes

            
            fault:		yes		can return with page locked

            
            page_mkwrite:	yes		can return with page locked

            
            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.


            	->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.


            	->access() is called when get_user_pages() fails in

            
            acces_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)