Re: [PATCH 9/9] mm, swap: use a single page for swap table when the size fits

[Chris Li <chrisl@kernel.org>]

Acked-by: Chris Li <chrisl@kernel.org>

Chris

On Fri, Aug 22, 2025 at 12:21 PM Kairui Song <ryncsn@gmail.com> wrote:
>
> From: Kairui Song <kasong@tencent.com>
>
> We have a cluster size of 512 slots. Each slot consumes 8 bytes in swap
> table so the swap table size of each cluster is exactly one page (4K).
>
> If that condition is true, allocate one page direct and disable the slab
> cache to reduce the memory usage of swap table and avoid fragmentation.
>
> Co-developed-by: Chris Li <chrisl@kernel.org>
> Signed-off-by: Chris Li <chrisl@kernel.org>
> Signed-off-by: Kairui Song <kasong@tencent.com>
> ---
>  mm/swap_table.h |  2 ++
>  mm/swapfile.c   | 50 ++++++++++++++++++++++++++++++++++++++++---------
>  2 files changed, 43 insertions(+), 9 deletions(-)
>
> diff --git a/mm/swap_table.h b/mm/swap_table.h
> index 4e97513b11ef..984474e37dd7 100644
> --- a/mm/swap_table.h
> +++ b/mm/swap_table.h
> @@ -11,6 +11,8 @@ struct swap_table {
>         atomic_long_t entries[SWAPFILE_CLUSTER];
>  };
>
> +#define SWP_TABLE_USE_PAGE (sizeof(struct swap_table) == PAGE_SIZE)
> +
>  /*
>   * A swap table entry represents the status of a swap slot on a swap
>   * (physical or virtual) device. The swap table in each cluster is a
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 00651e947eb2..7539ee26d59a 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -432,6 +432,38 @@ static inline unsigned int cluster_offset(struct swap_info_struct *si,
>         return cluster_index(si, ci) * SWAPFILE_CLUSTER;
>  }
>
> +static struct swap_table *swap_table_alloc(gfp_t gfp)
> +{
> +       struct folio *folio;
> +
> +       if (!SWP_TABLE_USE_PAGE)
> +               return kmem_cache_zalloc(swap_table_cachep, gfp);
> +
> +       folio = folio_alloc(gfp | __GFP_ZERO, 0);
> +       if (folio)
> +               return folio_address(folio);
> +       return NULL;
> +}
> +
> +static void swap_table_free_folio_rcu_cb(struct rcu_head *head)
> +{
> +       struct folio *folio;
> +
> +       folio = page_folio(container_of(head, struct page, rcu_head));
> +       folio_put(folio);
> +}
> +
> +static void swap_table_free(struct swap_table *table)
> +{
> +       if (!SWP_TABLE_USE_PAGE) {
> +               kmem_cache_free(swap_table_cachep, table);
> +               return;
> +       }
> +
> +       call_rcu(&(folio_page(virt_to_folio(table), 0)->rcu_head),
> +                swap_table_free_folio_rcu_cb);
> +}
> +
>  static void swap_cluster_free_table(struct swap_cluster_info *ci)
>  {
>         unsigned int ci_off;
> @@ -445,7 +477,7 @@ static void swap_cluster_free_table(struct swap_cluster_info *ci)
>         table = (void *)rcu_dereference_protected(ci->table, true);
>         rcu_assign_pointer(ci->table, NULL);
>
> -       kmem_cache_free(swap_table_cachep, table);
> +       swap_table_free(table);
>  }
>
>  /*
> @@ -469,8 +501,7 @@ swap_cluster_alloc_table(struct swap_info_struct *si,
>         lockdep_assert_held(&ci->lock);
>         lockdep_assert_held(&this_cpu_ptr(&percpu_swap_cluster)->lock);
>
> -       table = kmem_cache_zalloc(swap_table_cachep,
> -                                 __GFP_HIGH | __GFP_NOMEMALLOC | __GFP_NOWARN);
> +       table = swap_table_alloc(__GFP_HIGH | __GFP_NOMEMALLOC | __GFP_NOWARN);
>         if (table) {
>                 rcu_assign_pointer(ci->table, table);
>                 return ci;
> @@ -485,7 +516,7 @@ swap_cluster_alloc_table(struct swap_info_struct *si,
>         if (!(si->flags & SWP_SOLIDSTATE))
>                 spin_unlock(&si->global_cluster_lock);
>         local_unlock(&percpu_swap_cluster.lock);
> -       table = kmem_cache_zalloc(swap_table_cachep, __GFP_HIGH | GFP_KERNEL);
> +       table = swap_table_alloc(__GFP_HIGH | GFP_KERNEL);
>
>         local_lock(&percpu_swap_cluster.lock);
>         if (!(si->flags & SWP_SOLIDSTATE))
> @@ -522,7 +553,7 @@ swap_cluster_alloc_table(struct swap_info_struct *si,
>
>  free_table:
>         if (table)
> -               kmem_cache_free(swap_table_cachep, table);
> +               swap_table_free(table);
>         return ci;
>  }
>
> @@ -740,7 +771,7 @@ static int inc_cluster_info_page(struct swap_info_struct *si,
>
>         ci = cluster_info + idx;
>         if (!ci->table) {
> -               table = kmem_cache_zalloc(swap_table_cachep, GFP_KERNEL);
> +               table = swap_table_alloc(GFP_KERNEL);
>                 if (!table)
>                         return -ENOMEM;
>                 rcu_assign_pointer(ci->table, table);
> @@ -4076,9 +4107,10 @@ static int __init swapfile_init(void)
>          * only, and all swap cache readers (swap_cache_*) verifies
>          * the content before use. So it's safe to use RCU slab here.
>          */
> -       swap_table_cachep = kmem_cache_create("swap_table",
> -                           sizeof(struct swap_table),
> -                           0, SLAB_PANIC | SLAB_TYPESAFE_BY_RCU, NULL);
> +       if (!SWP_TABLE_USE_PAGE)
> +               swap_table_cachep = kmem_cache_create("swap_table",
> +                                   sizeof(struct swap_table),
> +                                   0, SLAB_PANIC | SLAB_TYPESAFE_BY_RCU, NULL);
>
>  #ifdef CONFIG_MIGRATION
>         if (swapfile_maximum_size >= (1UL << SWP_MIG_TOTAL_BITS))
> --
> 2.51.0
>