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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 6/19/26 00:17, Barry Song (Xiaomi) 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.
> 
> 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@xiaomi.com>
> Cc: David Hildenbrand <david@kernel.org>
> Cc: Lorenzo Stoakes <ljs@kernel.org>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Liam R. Howlett <liam@infradead.org>
> Cc: Nico Pache <npache@redhat.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Dev Jain <dev.jain@arm.com>
> Cc: Lance Yang <lance.yang@linux.dev>
> Cc: Kairui Song <kasong@tencent.com>
> Cc: Qi Zheng <qi.zheng@linux.dev>
> Cc: Shakeel Butt <shakeel.butt@linux.dev>
> Cc: Axel Rasmussen <axelrasmussen@google.com>
> Cc: Yuanchu Xie <yuanchu@google.com>
> Cc: Wei Xu <weixugc@google.com>
> Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
> ---
>  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;

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?

-- 
Cheers,

David