Re: [PATCH v2 1/9] mm: vmscan: add validation before spliting shmem large folio

[Baolin Wang]

On 2024/6/20 14:12, Yu Zhao wrote:
> On Tue, Jun 18, 2024 at 12:54 AM Baolin Wang
> <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
> > Add swap available space validation before spliting shmem large folio to
> > avoid redundant split, since we can not write shmem folio to the swap device
> > in this case.
>
> We don't scan anon LRU unless we know we may be able to swap. Even if
> there is swap space, we may still not be able to swap. See
> can_reclaim_anon_pages().

Right. Another case is MADV_PAGEOUT can still split a large shmem folio 
even without a swap device. I know this is not a common case, and I can 
drop this patch if it is less helpful.

Thanks for your input.

> > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > ---
> >   mm/vmscan.c | 8 ++++++++
> >   1 file changed, 8 insertions(+)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 1807e5d95dda..61465f92283f 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1237,6 +1237,14 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
> >                          }
> >                  } else if (folio_test_swapbacked(folio) &&
> >                             folio_test_large(folio)) {
> > +
> > +                       /*
> > +                        * Do not split shmem folio if no swap memory
> > +                        * available.
> > +                        */
> > +                       if (!total_swap_pages)
> > +                               goto activate_locked;
> > +
> >                          /* Split shmem folio */
> >                          if (split_folio_to_list(folio, folio_list))
> >                                  goto keep_locked;
> > --
> > 2.39.3