Re: [PATCH v3 1/3] mm, swap: speed up hibernation allocation and writeout

[Barry Song <21cnbao@gmail.com>]

On Mon, Feb 16, 2026 at 3:00 AM Kairui Song via B4 Relay
<devnull+kasong.tencent.com@kernel.org> wrote:
>
> From: Kairui Song <kasong@tencent.com>
>
> Since commit 0ff67f990bd4 ("mm, swap: remove swap slot cache"),
> hibernation has been using the swap slot slow allocation path for
> simplification, which turns out might cause regression for some
> devices because the allocator now rotates clusters too often, leading to
> slower allocation and more random distribution of data.
>
> Fast allocation is not complex, so implement hibernation support as
> well.
>
> Test result with Samsung SSD 830 Series (SATA II, 3.0 Gbps) shows the
> performance is several times better [1]:
> 6.19:               324 seconds
> After this series:  35 seconds
>
> Fixes: 0ff67f990bd4 ("mm, swap: remove swap slot cache")
> Reported-by: Carsten Grohmann <mail@carstengrohmann.de>
> Closes: https://lore.kernel.org/linux-mm/20260206121151.dea3633d1f0ded7bbf49c22e@linux-foundation.org/
> Link: https://lore.kernel.org/linux-mm/8b4bdcfa-ce3f-4e23-839f-31367df7c18f@gmx.de/ [1]
> Cc: stable@vger.kernel.org
> Signed-off-by: Kairui Song <kasong@tencent.com>
> ---
>  mm/swapfile.c | 21 ++++++++++++++++-----
>  1 file changed, 16 insertions(+), 5 deletions(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index c6863ff7152c..32e0e7545ab8 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1926,8 +1926,9 @@ void swap_put_entries_direct(swp_entry_t entry, int nr)
>  /* Allocate a slot for hibernation */
>  swp_entry_t swap_alloc_hibernation_slot(int type)
>  {
> -       struct swap_info_struct *si = swap_type_to_info(type);
> -       unsigned long offset;
> +       struct swap_info_struct *pcp_si, *si = swap_type_to_info(type);
> +       unsigned long pcp_offset, offset = SWAP_ENTRY_INVALID;
> +       struct swap_cluster_info *ci;
>         swp_entry_t entry = {0};
>
>         if (!si)
> @@ -1937,11 +1938,21 @@ swp_entry_t swap_alloc_hibernation_slot(int type)
>         if (get_swap_device_info(si)) {
>                 if (si->flags & SWP_WRITEOK) {
>                         /*
> -                        * Grab the local lock to be compliant
> -                        * with swap table allocation.
> +                        * Try the local cluster first if it matches the device. If
> +                        * not, try grab a new cluster and override local cluster.
>                          */
>                         local_lock(&percpu_swap_cluster.lock);
> -                       offset = cluster_alloc_swap_entry(si, NULL);
> +                       pcp_si = this_cpu_read(percpu_swap_cluster.si[0]);
> +                       pcp_offset = this_cpu_read(percpu_swap_cluster.offset[0]);
> +                       if (pcp_si == si && pcp_offset) {
> +                               ci = swap_cluster_lock(si, pcp_offset);
> +                               if (cluster_is_usable(ci, 0))
> +                                       offset = alloc_swap_scan_cluster(si, ci, NULL, pcp_offset);
> +                               else
> +                                       swap_cluster_unlock(ci);
> +                       }
> +                       if (!offset)

I assume you mean SWAP_ENTRY_INVALID? Would that be more readable?

> +                               offset = cluster_alloc_swap_entry(si, NULL);
>                         local_unlock(&percpu_swap_cluster.lock);
>                         if (offset)
>                                 entry = swp_entry(si->type, offset);
>
> --
> 2.52.0

Thanks
Barry