Re: [PATCH v3] vfs: don't unnecessarily clone write access for writable fds

[Al Viro]

On Mon, Jan 04, 2021 at 10:55:15AM -0800, Eric Biggers wrote:
> On Tue, Sep 22, 2020 at 09:44:18AM -0700, Eric Biggers wrote:
> > From: Eric Biggers <ebiggers@xxxxxxxxxx>
> > 
> > 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@xxxxxxxxxx>
> > ---
> > 
> > v3: added note to porting file.
> > v2: keep the check for emergency r/o remounts.
> > 
> >  Documentation/filesystems/porting.rst |  7 ++++
> >  fs/namespace.c                        | 53 ++++++++++-----------------
> >  include/linux/mount.h                 |  1 -
> >  3 files changed, 27 insertions(+), 34 deletions(-)
> 
> Ping.

Applied.