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

[Nico Pache]

On Thu, Jun 18, 2026 at 4:17 PM Barry Song (Xiaomi) <baohua@xxxxxxxxxx> wrote:
>
> When swap is disabled or exhausted, swap slot allocation
> may fail during swapout, causing large folios to be split
> into small folios. The splitting is reasonable when we
> truly fail to obtain contiguous swap slots, but it is
> pointless in the no-space case.

Hi Barry,

Are we sure splitting is pointless?  Since b1f202060afe ("mm: remap
unused subpages to shared zeropage when splitting isolated thp") a
split can lead to memory reclaim.

I've been far removed from the reclaim code for quite some time but im
assuming in this case we can actually reclaim some unused memory which
might be beneficial if there is memory pressure. However there are
some discussions on whether we should further guard this zeropage
remapping behavior [1].

[1] - https://lore.kernel.org/lkml/20260609114619.144416-1-npache@xxxxxxxxxx/

Cheers,
-- Nico

>
> A simple way to reproduce this is to invoke MADV_PAGEOUT on
> a system with mTHP enabled but without swap configured.
>
>  #define SIZE (16 * 1024 * 1024)
>  int main(void)
>  {
>          char *buf = mmap(NULL, SIZE, PROT_READ | PROT_WRITE,
>                     MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>          memset(buf, 1, SIZE);
>          madvise(buf, SIZE, MADV_PAGEOUT);
>          munmap(buf, SIZE);
>          return 0;
>  }
>
> With 16KB mTHP enabled, we observe:
> ~ # cat /sys/kernel/mm/transparent_hugepage/hugepages-16kB/stats/split
> 1024
>
> This patch checks swap space before splitting. If there is
> no available space, it skips splitting. After the patch, we
> observe:
> ~ # cat /sys/kernel/mm/transparent_hugepage/hugepages-16kB/stats/split
> 0
>
> Reported-by: Nanzhe Zhao <zhaonanzhe@xxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Lorenzo Stoakes <ljs@xxxxxxxxxx>
> Cc: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Cc: Liam R. Howlett <liam@xxxxxxxxxxxxx>
> Cc: Nico Pache <npache@xxxxxxxxxx>
> Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
> Cc: Dev Jain <dev.jain@xxxxxxx>
> Cc: Lance Yang <lance.yang@xxxxxxxxx>
> Cc: Kairui Song <kasong@xxxxxxxxxxx>
> Cc: Qi Zheng <qi.zheng@xxxxxxxxx>
> Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
> Cc: Yuanchu Xie <yuanchu@xxxxxxxxxx>
> Cc: Wei Xu <weixugc@xxxxxxxxxx>
> Signed-off-by: Barry Song (Xiaomi) <baohua@xxxxxxxxxx>
> ---
>  mm/vmscan.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 299b5d9e8836..33f84a5fe7ee 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -339,8 +339,7 @@ static bool can_demote(int nid, struct scan_control *sc,
>         return !nodes_empty(allowed_mask);
>  }
>
> -static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg,
> -                                         int nid,
> +static inline bool __can_reclaim_anon_pages(struct mem_cgroup *memcg,
>                                           struct scan_control *sc)
>  {
>         if (memcg == NULL) {
> @@ -356,6 +355,16 @@ static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg,
>                         return true;
>         }
>
> +       return false;
> +}
> +
> +static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg,
> +                                         int nid,
> +                                         struct scan_control *sc)
> +{
> +       if (__can_reclaim_anon_pages(memcg, sc))
> +               return true;
> +
>         /*
>          * 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;
>                                 /* Fallback to swap normal pages */
>                                 if (split_folio_to_list(folio, folio_list))
>                                         goto activate_locked;
> --
> 2.39.3 (Apple Git-146)
>