Re: [PATCH v5 9/9] mm: switch deferred split shrinker to list_lru

[Lorenzo Stoakes <ljs@kernel.org>]

On Tue, Jun 09, 2026 at 11:20:58AM +0800, Lance Yang wrote:
>
> On Mon, Jun 01, 2026 at 02:17:28PM -0400, Johannes Weiner wrote:
> >On Mon, Jun 01, 2026 at 09:21:35PM +0800, Lance Yang wrote:
> >>
> >> On Wed, May 27, 2026 at 04:45:16PM -0400, Johannes Weiner wrote:
> >> [...]
> >> >diff --git a/mm/swap_state.c b/mm/swap_state.c
> >> >index 04f5ce992401..9c3a5cf99778 100644
> >> >--- a/mm/swap_state.c
> >> >+++ b/mm/swap_state.c
> >> >@@ -465,6 +465,16 @@ static struct folio *__swap_cache_alloc(struct swap_cluster_info *ci,
> >> > 		return ERR_PTR(-ENOMEM);
> >> > 	}
> >> >
> >>
> >> Shouldn't this be limited to anon swapin?
> >>
> >> e.g. vmf && vma_is_anonymous(vmf->vma)
> >>
> >> >+	if (order > 1 && folio_memcg_alloc_deferred(folio)) {
> >>
> >> __swap_cache_alloc() is also used by shmem direct swapin, so shmem can
> >> get here too when handling a large swap entry:
> >>
> >> shmem_get_folio_gfp()
> >>   shmem_swapin_folio()
> >>     shmem_swap_alloc_folio()
> >>       swapin_sync()
> >>         swap_cache_alloc_folio()
> >>           __swap_cache_alloc()
> >>             folio_memcg_alloc_deferred()
> >
> >Good catch, I think you're right. I shouldn't have dismissed that
> >branch due to "/* Direct swapin skipping swap cache & readahead */"
> >
> >> @Baolin please correct me if I got it wrong :)
> >>
> >> folio_memcg_alloc_deferred() itself doesn't filter shmem out either; it
> >> only allocates the memcg list_lru metadata for deferred_split_lru:
> >>
> >> int folio_memcg_alloc_deferred(struct folio *folio)
> >> {
> >> 	if (mem_cgroup_disabled())
> >> 		return 0;
> >> 	return folio_memcg_list_lru_alloc(folio, &deferred_split_lru, GFP_KERNEL);
> >> }
> >>
> >> Since deferred_split_lru only queues anon large folios, doing this for
> >> shmem swapin doesn't buy us anything :)
> >
> >Yes, agreed. I don't think it's a big deal / show stopper in terms of
> >user-visible effect, but of course still worth fixing.
> >
> >I'll send a follow-up patch.
>
> Thanks.
>
> Looks like this has already landed in mm-stable. If you're okay with it,
> I can send the follow-up.
>
> From: Lance Yang <lance.yang@linux.dev>
> Date: Tue, 9 Jun 2026 10:56:45 +0800
> Subject: [PATCH] mm: prepare deferred split metadata only for anon swapin
>
> __swap_cache_alloc() prepares deferred split metadata for large swapcache
> folios.
>
> That also covers shmem swapin, because shmem_swap_alloc_folio() can call
> swapin_sync() with a large order[1]. But shmem folios are not queued on
> the deferred split queue, so preparing the metadata doesn't buy us
> anything there.
>
> So let's limit it to anon swapin.
>
> [1] https://lore.kernel.org/all/20260601132135.14272-1-lance.yang@linux.dev/
>
> Signed-off-by: Lance Yang <lance.yang@linux.dev>
> ---
>  mm/swap_state.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 9c3a5cf99778..7adac957c2b8 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -465,7 +465,8 @@ static struct folio *__swap_cache_alloc(struct swap_cluster_info *ci,
>  		return ERR_PTR(-ENOMEM);
>  	}
>
> -	if (order > 1 && folio_memcg_alloc_deferred(folio)) {
> +	if (order > 1 && vma && vma_is_anonymous(vma) &&

A folio can be anon for a non-shmem file-backed VMA though?
E.g. MAP_PRIVATE-mapped file-backed mappings?

Not sure if that's something that'd be a factor here/meaningful though.

> +	    folio_memcg_alloc_deferred(folio)) {
>  		spin_lock(&ci->lock);
>  		__swap_cache_do_del_folio(ci, folio, entry, shadow);
>  		spin_unlock(&ci->lock);
> --
> 2.39.3 (Apple Git-146)

Cheers, Lorenzo