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Message-ID: <CACePvbWSsy4BzJuWOHr5k_g9D3jqjevdWtwPoTg88H0O+noE0A@mail.gmail.com>
Date: Fri, 29 Aug 2025 21:23:23 -0700
From: Chris Li <chrisl@...nel.org>
To: Kairui Song <kasong@...cent.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
Matthew Wilcox <willy@...radead.org>, Hugh Dickins <hughd@...gle.com>, Barry Song <baohua@...nel.org>,
Baoquan He <bhe@...hat.com>, Nhat Pham <nphamcs@...il.com>,
Kemeng Shi <shikemeng@...weicloud.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
Ying Huang <ying.huang@...ux.alibaba.com>, Johannes Weiner <hannes@...xchg.org>,
David Hildenbrand <david@...hat.com>, Yosry Ahmed <yosryahmed@...gle.com>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Zi Yan <ziy@...dia.com>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 9/9] mm, swap: use a single page for swap table when the
size fits
Acked-by: Chris Li <chrisl@...nel.org>
Chris
On Fri, Aug 22, 2025 at 12:21 PM Kairui Song <ryncsn@...il.com> wrote:
>
> From: Kairui Song <kasong@...cent.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@...nel.org>
> Signed-off-by: Chris Li <chrisl@...nel.org>
> Signed-off-by: Kairui Song <kasong@...cent.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
>
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