Re: [PATCH v2 4/6] shmem: skip page split if we're not reclaiming

[Luis Chamberlain <mcgrof@kernel.org>]

On Mon, Apr 17, 2023 at 09:41:41PM -0700, Hugh Dickins wrote:
> On Thu, 9 Mar 2023, Luis Chamberlain wrote:
> 
> > In theory when info->flags & VM_LOCKED we should not be getting
> > shem_writepage() called so we should be verifying this with a
> > WARN_ON_ONCE(). Since we should not be swapping then best to ensure
> > we also don't do the folio split earlier too. So just move the check
> > early to avoid folio splits in case its a dubious call.
> > 
> > We also have a similar early bail when !total_swap_pages so just move
> > that earlier to avoid the possible folio split in the same situation.
> > 
> > Acked-by: David Hildenbrand <david@redhat.com>
> > Reviewed-by: Christian Brauner <brauner@kernel.org>
> > Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
> > ---
> >  mm/shmem.c | 10 ++++++----
> >  1 file changed, 6 insertions(+), 4 deletions(-)
> > 
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index 68e9970baf1e..dfd995da77b4 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -1350,6 +1350,12 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
> >  	if (WARN_ON_ONCE(!wbc->for_reclaim))
> >  		goto redirty;
> >  
> > +	if (WARN_ON_ONCE(info->flags & VM_LOCKED))
> > +		goto redirty;
> 
> Well, okay, I don't mind that.  But shall we take bets on how soon syzbot
> (hope it's not watching) will try flipping SHM_LOCK on while swapping out
> pages from a SHM segment, and hit that warning?  Perhaps I'm wrong, but I
> don't think any serialization prevents that.

I though that may be the case. Would such serialization be welcomed?

  Luis