Re: [PATCH 10/19] mm, swap: consolidate cluster reclaim and check logic

[Kairui Song <ryncsn@gmail.com>]

On Fri, Oct 31, 2025 at 1:25 PM YoungJun Park <youngjun.park@lge.com> wrote:
>
> On Wed, Oct 29, 2025 at 11:58:36PM +0800, Kairui Song wrote:
>
> > From: Kairui Song <kasong@tencent.com>
> >
>
> Hello Kairu, great work on your patchwork. :)
> > Swap cluster cache reclaim requires releasing the lock, so some extra
> > checks are needed after the reclaim. To prepare for checking swap cache
> > using the swap table directly, consolidate the swap cluster reclaim and
> > check the logic.
> >
> > Also, adjust it very slightly. By moving the cluster empty and usable
> > check into the reclaim helper, it will avoid a redundant scan of the
> > slots if the cluster is empty.
>
> This is Change 1
>
> > And always scan the whole region during reclaim, don't skip slots
> > covered by a reclaimed folio. Because the reclaim is lockless, it's
> > possible that new cache lands at any time. And for allocation, we want
> > all caches to be reclaimed to avoid fragmentation. And besides, if the
> > scan offset is not aligned with the size of the reclaimed folio, we are
> > skipping some existing caches.
>
> This is Change 2
>
> > There should be no observable behavior change, which might slightly
> > improve the fragmentation issue or performance.
> >
> > Signed-off-by: Kairui Song <kasong@tencent.com>
> > ---
> >  mm/swapfile.c | 47 +++++++++++++++++++++++------------------------
> >  1 file changed, 23 insertions(+), 24 deletions(-)
> >
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index d66141f1c452..e4c521528817 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -778,42 +778,50 @@ static int swap_cluster_setup_bad_slot(struct swap_cluster_info *cluster_info,
> >       return 0;
> >  }
> >
> > -static bool cluster_reclaim_range(struct swap_info_struct *si,
> > -                               struct swap_cluster_info *ci,
> > -                               unsigned long start, unsigned long end)
> > +static unsigned int cluster_reclaim_range(struct swap_info_struct *si,
> > +                                       struct swap_cluster_info *ci,
> > +                                       unsigned long start, unsigned int order)
> >  {
> > +     unsigned int nr_pages = 1 << order;
> > +     unsigned long offset = start, end = start + nr_pages;
> >       unsigned char *map = si->swap_map;
> > -     unsigned long offset = start;
> >       int nr_reclaim;
> >
> >       spin_unlock(&ci->lock);
> >       do {
> >               switch (READ_ONCE(map[offset])) {
> >               case 0:
> > -                     offset++;
> >                       break;
> >               case SWAP_HAS_CACHE:
> >                       nr_reclaim = __try_to_reclaim_swap(si, offset, TTRS_ANYWAY);
> > -                     if (nr_reclaim > 0)
> > -                             offset += nr_reclaim;
> > -                     else
> > +                     if (nr_reclaim < 0)
> >                               goto out;
> >                       break;
> >               default:
> >                       goto out;
> >               }
> > -     } while (offset < end);
> > +     } while (++offset < end);
>
> Change 2
>
> >  out:
> >       spin_lock(&ci->lock);
> > +
> > +     /*
> > +      * We just dropped ci->lock so cluster could be used by another
> > +      * order or got freed, check if it's still usable or empty.
> > +      */
> > +     if (!cluster_is_usable(ci, order))
> > +             return SWAP_ENTRY_INVALID;
> > +     if (cluster_is_empty(ci))
> > +             return cluster_offset(si, ci);
> > +
>
> Change 1
>
> >       /*
> >        * Recheck the range no matter reclaim succeeded or not, the slot
> >        * could have been be freed while we are not holding the lock.
> >        */
> >       for (offset = start; offset < end; offset++)
> >               if (READ_ONCE(map[offset]))
> > -                     return false;
> > +                     return SWAP_ENTRY_INVALID;
> >
> > -     return true;
> > +     return start;
> >  }
> >
> >  static bool cluster_scan_range(struct swap_info_struct *si,
> > @@ -901,7 +909,7 @@ static unsigned int alloc_swap_scan_cluster(struct swap_info_struct *si,
> >       unsigned long start = ALIGN_DOWN(offset, SWAPFILE_CLUSTER);
> >       unsigned long end = min(start + SWAPFILE_CLUSTER, si->max);
> >       unsigned int nr_pages = 1 << order;
> > -     bool need_reclaim, ret;
> > +     bool need_reclaim;
> >
> >       lockdep_assert_held(&ci->lock);
> >
> > @@ -913,20 +921,11 @@ static unsigned int alloc_swap_scan_cluster(struct swap_info_struct *si,
> >               if (!cluster_scan_range(si, ci, offset, nr_pages, &need_reclaim))
> >                       continue;
> >               if (need_reclaim) {
> > -                     ret = cluster_reclaim_range(si, ci, offset, offset + nr_pages);
> > -                     /*
> > -                      * Reclaim drops ci->lock and cluster could be used
> > -                      * by another order. Not checking flag as off-list
> > -                      * cluster has no flag set, and change of list
> > -                      * won't cause fragmentation.
> > -                      */
> > -                     if (!cluster_is_usable(ci, order))
> > -                             goto out;
> > -                     if (cluster_is_empty(ci))
> > -                             offset = start;
> > +                     found = cluster_reclaim_range(si, ci, offset, order);
> >                       /* Reclaim failed but cluster is usable, try next */
> > -                     if (!ret)
>
> Part of Change 1 (apply return value change)
>
> As I understand Change 1 just remove redudant checking.
> But, I think another part changed also.
> (maybe I don't fully understand comment or something)
>
> cluster_reclaim_range can return SWAP_ENTRY_INVALID
> if the cluster becomes unusable for the requested order.
> (!cluster_is_usable return SWAP_ENTRY_INVALID)
> And it continues loop to the next offset for reclaim try.
> Is this the intended behavior?

Thanks for the very careful review! I should keep the
cluster_is_usable check or abort in other ways to avoid touching an
unusable cluster, will fix it.