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

[Barry Song (Xiaomi)]

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;
 	}
 
 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))) {
 				int __maybe_unused order = folio_order(folio);
 
-				if (!folio_test_large(folio))
+				if (!folio_test_large(folio) || ret != -EAGAIN)
 					goto activate_locked_split;
 				/* Fallback to swap normal pages */
 				if (split_folio_to_list(folio, folio_list))

What’s your view on this, David?

[1] https://lore.kernel.org/linux-mm/CAMgjq7Bmi2XYxPc4tVO8KTWSuj8jpt-c-JueqYRK1kLC_nmBqA@xxxxxxxxxxxxxx/

Best Regards
Barry