Commit 14e43bf4 authored by Eric Biggers's avatar Eric Biggers Committed by Al Viro
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vfs: don't unnecessarily clone write access for writable fds 



There's no need for mnt_want_write_file() to increment mnt_writers when
the file is already open for writing, provided that
mnt_drop_write_file() is changed to conditionally decrement it.

We seem to have ended up in the current situation because
mnt_want_write_file() used to be paired with mnt_drop_write(), due to
mnt_drop_write_file() not having been added yet.  So originally
mnt_want_write_file() had to always increment mnt_writers.

But later mnt_drop_write_file() was added, and all callers of
mnt_want_write_file() were paired with it.  This makes the compatibility
between mnt_want_write_file() and mnt_drop_write() no longer necessary.

Therefore, make __mnt_want_write_file() and __mnt_drop_write_file() skip
incrementing mnt_writers on files already open for writing.  This
removes the only caller of mnt_clone_write(), so remove that too.

Signed-off-by: default avatarEric Biggers <ebiggers@google.com>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 5c8fe583

Documentation/filesystems/porting.rst

+7 −0
Original line number Diff line number Diff line
@@ -865,3 +865,10 @@ no matter what. Everything is handled by the caller. 


clone_private_mount() returns a longterm mount now, so the proper destructor of

its result is kern_unmount() or kern_unmount_array().



---



**mandatory**



mnt_want_write_file() can now only be paired with mnt_drop_write_file(),

whereas previously it could be paired with mnt_drop_write() as well.

fs/namespace.c

+20 −33
Original line number Diff line number Diff line
@@ -359,51 +359,37 @@ int mnt_want_write(struct vfsmount *m) 
}

EXPORT_SYMBOL_GPL(mnt_want_write);



/**

 * mnt_clone_write - get write access to a mount

 * @mnt: the mount on which to take a write

 *

 * This is effectively like mnt_want_write, except

 * it must only be used to take an extra write reference

 * on a mountpoint that we already know has a write reference

 * on it. This allows some optimisation.

 *

 * After finished, mnt_drop_write must be called as usual to

 * drop the reference.

 */

int mnt_clone_write(struct vfsmount *mnt)

{

	/* superblock may be r/o */

	if (__mnt_is_readonly(mnt))

		return -EROFS;

	preempt_disable();

	mnt_inc_writers(real_mount(mnt));

	preempt_enable();

	return 0;

}

EXPORT_SYMBOL_GPL(mnt_clone_write);



/**

 * __mnt_want_write_file - get write access to a file's mount

 * @file: the file who's mount on which to take a write

 *

 * This is like __mnt_want_write, but it takes a file and can

 * do some optimisations if the file is open for write already

 * This is like __mnt_want_write, but if the file is already open for writing it

 * skips incrementing mnt_writers (since the open file already has a reference)

 * and instead only does the check for emergency r/o remounts.  This must be

 * paired with __mnt_drop_write_file.

 */

int __mnt_want_write_file(struct file *file)

{

	if (!(file->f_mode & FMODE_WRITER))

	if (file->f_mode & FMODE_WRITER) {

		/*

		 * Superblock may have become readonly while there are still

		 * writable fd's, e.g. due to a fs error with errors=remount-ro

		 */

		if (__mnt_is_readonly(file->f_path.mnt))

			return -EROFS;

		return 0;

	}

	return __mnt_want_write(file->f_path.mnt);

	else

		return mnt_clone_write(file->f_path.mnt);

}



/**

 * mnt_want_write_file - get write access to a file's mount

 * @file: the file who's mount on which to take a write

 *

 * This is like mnt_want_write, but it takes a file and can

 * do some optimisations if the file is open for write already

 * This is like mnt_want_write, but if the file is already open for writing it

 * skips incrementing mnt_writers (since the open file already has a reference)

 * and instead only does the freeze protection and the check for emergency r/o

 * remounts.  This must be paired with mnt_drop_write_file.

 */

int mnt_want_write_file(struct file *file)

{
@@ -449,6 +435,7 @@ EXPORT_SYMBOL_GPL(mnt_drop_write); 


void __mnt_drop_write_file(struct file *file)

{

	if (!(file->f_mode & FMODE_WRITER))

		__mnt_drop_write(file->f_path.mnt);

}

include/linux/mount.h

+0 −1
Original line number Diff line number Diff line
@@ -79,7 +79,6 @@ struct path; 


extern int mnt_want_write(struct vfsmount *mnt);

extern int mnt_want_write_file(struct file *file);

extern int mnt_clone_write(struct vfsmount *mnt);

extern void mnt_drop_write(struct vfsmount *mnt);

extern void mnt_drop_write_file(struct file *file);

extern void mntput(struct vfsmount *mnt);