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vfs: don't unnecessarily clone write access for writable fds

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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: Eric Biggers <ebiggers@google.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Eric Biggers 2020-09-22 09:44:18 -07:00 • committed by Al Viro
parent 5c8fe583cc
commit 14e43bf435
3 changed files with 27 additions and 34 deletions
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1
include/linux/mount.h
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@ -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);