Re: [PATCH 2/2] mm, swap: prefer nonfull over free clusters

[Nhat Pham <nphamcs@gmail.com>]

On Mon, Aug 4, 2025 at 10:24 AM Kairui Song <ryncsn@gmail.com> wrote:
>
> From: Kairui Song <kasong@tencent.com>
>
> We prefer a free cluster over a nonfull cluster whenever a CPU local
> cluster is drained to respect the SSD discard behavior [1]. It's not
> a best practice for non-discarding devices. And this is causing a
> chigher fragmentation rate.
>
> So for a non-discarding device, prefer nonfull over free clusters. This
> reduces the fragmentation issue by a lot.
>
> Testing with make -j96, defconfig, using 64k mTHP, 8G ZRAM:
>
> Before: sys time: 6121.0s  64kB/swpout: 1638155  64kB/swpout_fallback: 189562
> After:  sys time: 6145.3s  64kB/swpout: 1761110  64kB/swpout_fallback: 66071
>
> Testing with make -j96, defconfig, using 64k mTHP, 10G ZRAM:
>
> Before: sys time 5527.9s  64kB/swpout: 1789358  64kB/swpout_fallback: 17813
> After:  sys time 5538.3s  64kB/swpout: 1813133  64kB/swpout_fallback: 0
>
> Performance is basically unchanged, and the large allocation failure rate
> is lower. Enabling all mTHP sizes showed a more significant result:
>
> Using the same test setup with 10G ZRAM and enabling all mTHP sizes:
>
> 128kB swap failure rate:
> Before: swpout:449548 swpout_fallback:55894
> After:  swpout:497519 swpout_fallback:3204
>
> 256kB swap failure rate:
> Before: swpout:63938  swpout_fallback:2154
> After:  swpout:65698  swpout_fallback:324
>
> 512kB swap failure rate:
> Before: swpout:11971  swpout_fallback:2218
> After:  swpout:14606  swpout_fallback:4
>
> 2M swap failure rate:
> Before: swpout:12     swpout_fallback:1578
> After:  swpout:1253   swpout_fallback:15
>
> The success rate of large allocations is much higher.
>
> Link: https://lore.kernel.org/linux-mm/87v8242vng.fsf@yhuang6-desk2.ccr.corp.intel.com/ [1]
> Signed-off-by: Kairui Song <kasong@tencent.com>

Nice! I agree with Chris' analysis too. It's less of a problem for
vswap (because there's no physical/SSD implication over there), but
this patch makes sense in the context of swapfile allocator.

FWIW:
Reviewed-by: Nhat Pham <nphamcs@gmail.com>

> ---
>  mm/swapfile.c | 38 ++++++++++++++++++++++++++++----------
>  1 file changed, 28 insertions(+), 10 deletions(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 5fdb3cb2b8b7..4a0cf4fb348d 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -908,18 +908,20 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si, int o
>         }
>
>  new_cluster:
> -       ci = isolate_lock_cluster(si, &si->free_clusters);
> -       if (ci) {
> -               found = alloc_swap_scan_cluster(si, ci, cluster_offset(si, ci),
> -                                               order, usage);
> -               if (found)
> -                       goto done;
> +       /*
> +        * If the device need discard, prefer new cluster over nonfull
> +        * to spread out the writes.
> +        */
> +       if (si->flags & SWP_PAGE_DISCARD) {
> +               ci = isolate_lock_cluster(si, &si->free_clusters);
> +               if (ci) {
> +                       found = alloc_swap_scan_cluster(si, ci, cluster_offset(si, ci),
> +                                                       order, usage);
> +                       if (found)
> +                               goto done;
> +               }
>         }
>
> -       /* Try reclaim from full clusters if free clusters list is drained */
> -       if (vm_swap_full())
> -               swap_reclaim_full_clusters(si, false);
> -
>         if (order < PMD_ORDER) {
>                 while ((ci = isolate_lock_cluster(si, &si->nonfull_clusters[order]))) {
>                         found = alloc_swap_scan_cluster(si, ci, cluster_offset(si, ci),
> @@ -927,7 +929,23 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si, int o
>                         if (found)
>                                 goto done;
>                 }
> +       }
>
> +       if (!(si->flags & SWP_PAGE_DISCARD)) {
> +               ci = isolate_lock_cluster(si, &si->free_clusters);
> +               if (ci) {
> +                       found = alloc_swap_scan_cluster(si, ci, cluster_offset(si, ci),
> +                                                       order, usage);
> +                       if (found)
> +                               goto done;
> +               }
> +       }

Seems like this pattern is repeated a couple of places -
isolate_lock_cluster from one of the lists, and if successful, then
try to allocate (alloc_swap_scan_cluster) from it.

Might be refactorable in a future clean up patch.