Re: [RFC PATCH] mm: Avoiding split large folios if swap has no space

[Barry Song]

On Mon, Jun 22, 2026 at 11:04 AM Baolin Wang
<baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
>
>
>
> On 6/20/26 4:10 PM, Barry Song (Xiaomi) wrote:
> > On Fri, Jun 19, 2026 at 10:04 PM David Hildenbrand (Arm) <david@xxxxxxxxxx> wrote:
> > [...]
> >>>        /*
> >>>         * The page can not be swapped.
> >>>         *
> >>> @@ -1280,6 +1289,8 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
> >>>
> >>>                                if (!folio_test_large(folio))
> >>>                                        goto activate_locked_split;
> >>> +                             if (!__can_reclaim_anon_pages(memcg, sc))
> >>> +                                     goto activate_locked_split;
> >>
> >> Why are we even trying to allocate swap space if we cannot reclaim such pages?
> >> Makes we wonder whether we would want to have that check earlier, before the
> >> folio_alloc_swap().
> >>
> >> Any downsides?
> >
> > I don't think there are any obvious downsides there. One issue is that
> > the memcg may not be passed from reclaim_pages(), so memcg would
> > always be NULL. However, the folio could still belong to a memcg
> > whose swap quota has been exhausted. In that case, my
> > __can_reclaim_anon_pages() will fail when checking whether we can
> > swap out. But switching to folio_memcg() also seems awkward.
> >
> > So I feel Kairui’s suggestion [1] might be the best approach. In
> > folio_alloc_swap(), we return -EAGAIN to tell vmscan.c that
> > we can split the folio and retry the swap-out.
> > only when there are sufficient swap slots and sufficient memcg swap
> > quota do we return -EAGAIN, allowing vmscan to perform a split.
> >
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index 78b49b0658ad..62e2c506ccae 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -1755,6 +1755,9 @@ int folio_alloc_swap(struct folio *folio)
> >                       VM_WARN_ON_ONCE(1);
> >                       return -EINVAL;
> >               }
> > +
> > +             if (get_nr_swap_pages() < (1 << order))
> > +                     return -ENOMEM;
>
> Shouldn't this return -EAGAIN? Suppose we try to swap out an order-9
> large folio but get_nr_swap_pages() returns 256, then we'd still need to
> split the order-9 large folio to reclaim some memory.

I guess Kairui has the opposite view. Quoting Kairui:

"- 1. the mem_cgroup_try_charge_swap in it failed
- 2. allocation failed but nr_swap_pages > folio size
- 3. allocation failed because all devices are full or unusable
(roughly nr_swap_pages < folio size)

Only case 2 requires splitting. __can_reclaim_anon_pages also checks
demote which is not related to swap."

For example, if get_nr_swap_pages() returns 1, we can only
swap out a single page after splitting. Is that really worth
it? On the other hand, if it returns 256, we could at least
swap out half of an order-9 large folio. Isn't that worthwhile?

>
> >       }
> >
> >   again:
> > @@ -1769,11 +1772,13 @@ int folio_alloc_swap(struct folio *folio)
> >       }
> >
> >       /* Need to call this even if allocation failed, for MEMCG_SWAP_FAIL. */
> > -     if (unlikely(mem_cgroup_try_charge_swap(folio)))
> > +     if (unlikely(mem_cgroup_try_charge_swap(folio))) {
> >               swap_cache_del_folio(folio);
> > +             return -ENOMEM;
> > +     }
> >
> >       if (unlikely(!folio_test_swapcache(folio)))
> > -             return -ENOMEM;
> > +             return -EAGAIN;
> >
> >       return 0;
> >   }
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 299b5d9e8836..63e8578454ea 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1257,6 +1257,8 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
> >                */
> >               if (folio_test_anon(folio) && folio_test_swapbacked(folio) &&
> >                               !folio_test_swapcache(folio)) {
> > +                     int ret;
> > +
> >                       if (!(sc->gfp_mask & __GFP_IO))
> >                               goto keep_locked;
> >                       if (folio_maybe_dma_pinned(folio))
> > @@ -1275,10 +1277,10 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
> >                                   split_folio_to_list(folio, folio_list))
> >                                       goto activate_locked;
> >                       }
> > -                     if (folio_alloc_swap(folio)) {
> > +                     if ((ret = folio_alloc_swap(folio))) {
>
> Also, please give shmem some love (shmem also calls folio_alloc_swap()
> when swapping out) :)

Sure. I'm pretty sure I love shmem. :-)

Best Regards
Barry