Re: [PATCH v5 9/9] mm: switch deferred split shrinker to list_lru
From: Lance Yang
Date: Tue Jun 09 2026 - 04:06:17 EST
On 2026/6/9 15:35, Lorenzo Stoakes wrote:
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@xxxxxxxxx>
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@xxxxxxxxx/
Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
---
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?
Ah, good point! vma_is_anonymous() is too strong here ...
Maybe use !vma_is_shmem(vma) instead?
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