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

[Baolin Wang]

On 6/20/26 4:10 PM, Barry Song (Xiaomi) wrote:
> 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;

Shouldn't this return -EAGAIN? Suppose we try to swap out an order-9 
large folio but get_nr_swap_pages() returns 256, then we'd still need to 
split the order-9 large folio to reclaim some memory.

>   	}
>   
>   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))) {

Also, please give shmem some love (shmem also calls folio_alloc_swap() 
when swapping out) :)