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

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