On Fri, Sep 29, 2023 at 03:31:29PM +0200, Jann Horn wrote:
> On Fri, Sep 29, 2023 at 11:20 AM Christian Brauner <brauner@kernel.org> wrote:
> > > But yes, that protection would be broken by SLAB_TYPESAFE_BY_RCU,
> > > since then the "f_count is zero" is no longer a final thing.
> >
> > I've tried coming up with a patch that is simple enough so the pattern
> > is easy to follow and then converting all places to rely on a pattern
> > that combine lookup_fd_rcu() or similar with get_file_rcu(). The obvious
> > thing is that we'll force a few places to now always acquire a reference
> > when they don't really need one right now and that already may cause
> > performance issues.
> 
> (Those places are probably used way less often than the hot
> open/fget/close paths though.)
> 
> > We also can't fully get rid of plain get_file_rcu() uses itself because
> > of users such as mm->exe_file. They don't go from one of the rcu fdtable
> > lookup helpers to the struct file obviously. They rcu replace the file
> > pointer in their struct ofc so we could change get_file_rcu() to take a
> > struct file __rcu **f and then comparing that the passed in pointer
> > hasn't changed before we managed to do atomic_long_inc_not_zero(). Which
> > afaict should work for such cases.
> >
> > But overall we would introduce a fairly big and at the same time subtle
> > semantic change. The idea is pretty neat and it was fun to do but I'm
> > just not convinced we should do it given how ubiquitous struct file is
> > used and now to make the semanics even more special by allowing
> > refcounts.
> >
> > I've kept your original release_empty_file() proposal in vfs.misc which
> > I think is a really nice change.
> >
> > Let me know if you all passionately disagree. ;)

So I'm appending the patch I had played with and a fix from Jann on top.
@Linus, if you have an opinion, let me know what you think.

Also available here:
https://gitlab.com/brauner/linux/-/commits/vfs.file.rcu

Might be interesting if this could be perfed to see if there is any real
gain for workloads with massive numbers of fds.