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

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Sep 22, 2020 at 09:44:18AM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> 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>
> ---
> 
> 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.