Re: [PATCH v2 1/1] mm, swap: only prefer nonfull clusters on synchronous devices

[Alexandre Ghiti]

Hi Andrew,

On Tue, Nov 10, 2026 at 12:24 AM Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> >
> On Mon, 9 Nov 2026 10:00:45 +0100 Alexandre Ghiti <alexghiti@xxxxxxxx> wrote:
>
> > > The patch itself looks good to me.
> > >
> > > Acked-by: Kairui Song <kasong@xxxxxxxxxxx>
> >
> > Thanks,
>
> Thanks all.
>
> Please avoid single-patch patch series?  Present them as a unified
> singleton.

Sorry, won't do again.

>
> Here's what I ended up with.  Please check.
>
>
> From: Alexandre Ghiti <alex@xxxxxxxx>
> Subject: mm, swap: only prefer nonfull clusters on synchronous devices
> Date: Mon, 09 Nov 2026 15:18:32 -0800
>
> Commit 9a42aed48421 ("mm, swap: prefer nonfull over free clusters") made
> the allocator fill nonfull clusters before taking free ones on swap
> devices without discard.  Commit 3cedb745d354 ("mm: clarify the
> folio_free_swap() for do_swap_page()") frees the swap slot on an exclusive
> write fault.  Such faults now leave holes in formerly full clusters,
> putting those clusters back on the nonfull list.  Reclaim allocates slots
> one folio at a time, scattering a batch across the holes instead of
> producing mergeable writes.  Later swapin can also require more read
> requests.
>
> On a mysql/sysbench workload in a 256 MiB memcg swapping to an NVMe
> partition, 7.3 compared with the previous version produced 59% more device
> read requests and 27% more write requests per transaction.  Throughput was
> unchanged without iocost and fell 19% with iocost.
>
> Prefer free clusters first on nonsynchronous devices, preserving
> contiguous swap slots for merging I/O.  Keep nonfull-first on synchronous
> devices such as zram: they do not merge I/O, and filling holes saves free
> clusters for large folios.  Keep free-cluster preference on discard
> devices to spread writes.
>
> In the same workload, v2 reduced read requests per transaction by 14% and
> write requests by 48% against its base kernel.  Throughput was unchanged
> without iocost and improved 33% with iocost.  The RFC's batch-aware
> allocator also improved this workload, but this policy needs no batch
> information from reclaim.  Reverting the slot-freeing change would keep
> swap space pinned under dirty pages; a proposed 50% swap-usage cutoff for
> free-cluster preference was arbitrary and would stop helping as swap
> filled.
>
> The 7.0 to 7.2 kernels retained the swap slot after a write fault as a
> side effect of 4b34f1d82c65 ("mm, swap: free the swap cache after folio is
> mapped").  The observed regression requires both 9a42aed48421 and
> 3cedb745d354; stable backports should target trees containing both.

That's fine by me!

Thanks,

Alex

>
> Link: https://lore.kernel.org/r/20261105155032.240457-1-alex@xxxxxxxx
> Link: https://lore.kernel.org/r/20261105155032.240457-2-alex@xxxxxxxx
> Link: https://lore.kernel.org/r/20261022145615.2184343-1-alex@xxxxxxxx
> Link: https://lore.kernel.org/r/CAMgjq7BFbXEKEt9faj_rg5AStfLkpJ0j57EQOOJ+L3-w2xRvtg@xxxxxxxxxxxxxx
> Fixes: 9a42aed48421 ("mm, swap: prefer nonfull over free clusters")
> Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Suggested-by: Kairui Song <kasong@xxxxxxxxxxx>
> Acked-by: Kairui Song <kasong@xxxxxxxxxxx>
> Reviewed-by: Nhat Pham <nphamcs@xxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> ---
>
>  mm/swapfile.c |   19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
>
> --- a/mm/swapfile.c~mm-swap-only-prefer-nonfull-clusters-on-synchronous-devices
> +++ a/mm/swapfile.c
> @@ -973,6 +973,17 @@ static bool __swap_cluster_alloc_entries
>         return true;
>  }
>
> +/*
> + * Prefer a new cluster over a nonfull one if the device needs discard, to
> + * spread out the writes, or if it is not synchronous, to keep the slots
> + * of a swapout batch contiguous so that its writes can be merged.
> + */
> +static bool swap_prefer_free_cluster(struct swap_info_struct *si)
> +{
> +       return (si->flags & SWP_PAGE_DISCARD) ||
> +              !(si->flags & SWP_SYNCHRONOUS_IO);
> +}
> +
>  /* Try use a new cluster for current CPU and allocate from it. */
>  static unsigned int alloc_swap_scan_cluster(struct swap_info_struct *si,
>                                             struct swap_cluster_info *ci,
> @@ -1143,11 +1154,7 @@ static unsigned long cluster_alloc_swap_
>         }
>
>  new_cluster:
> -       /*
> -        * If the device need discard, prefer new cluster over nonfull
> -        * to spread out the writes.
> -        */
> -       if (si->flags & SWP_PAGE_DISCARD) {
> +       if (swap_prefer_free_cluster(si)) {
>                 found = alloc_swap_scan_list(si, &si->free_clusters, folio, false);
>                 if (found)
>                         goto done;
> @@ -1159,7 +1166,7 @@ new_cluster:
>                         goto done;
>         }
>
> -       if (!(si->flags & SWP_PAGE_DISCARD)) {
> +       if (!swap_prefer_free_cluster(si)) {
>                 found = alloc_swap_scan_list(si, &si->free_clusters, folio, false);
>                 if (found)
>                         goto done;
> _
>