Re: [PATCH v2] vfs: don't unnecessarily clone write access for writable fds

[Eric Biggers <ebiggers@kernel.org>]

On Thu, Sep 17, 2020 at 01:54:41AM +0100, Al Viro wrote:
> On Tue, Sep 15, 2020 at 08:59:14PM -0700, Eric Biggers wrote:
> > On Mon, Jun 29, 2020 at 09:50:14AM -0700, Eric Biggers wrote:
> > > On Thu, Jun 11, 2020 at 09:05:34AM -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.
> 
> Umm...  That really needs to be put into D/f/porting; this kind of rule changes
> (from "it used to work both ways" to "things quietly break if you use the
> old variant") should come with explicit statement in there.
> 
> I'm certainly fine with unexporting mnt_clone_write() and making the damn
> thing static, but as for the rest I would put an explicit "don't pair
> mnt_drop_write() with mnt_want_write_file()" and wait for a cycle.

Is there any point in waiting a cycle between adding the note to
Documentation/filesystems/porting.rst and making the behavior change?  It seems
that all the other notes just get added at the same time the change is made.

- Eric