linux-kernel - Re: [PATCH 16/19] mm, swap: check swap table directly for checking cache

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Message-ID: <CAGsJ_4zW92dnCk6qUF9fKe+=b0dORLrBjuQC5mjq_nbTK4aT4g@mail.gmail.com>
Date: Fri, 7 Nov 2025 05:02:59 +0800
From: Barry Song <21cnbao@...il.com>
To: Kairui Song <ryncsn@...il.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, 
	Baoquan He <bhe@...hat.com>, Chris Li <chrisl@...nel.org>, Nhat Pham <nphamcs@...il.com>, 
	Johannes Weiner <hannes@...xchg.org>, Yosry Ahmed <yosry.ahmed@...ux.dev>, 
	David Hildenbrand <david@...hat.com>, Youngjun Park <youngjun.park@....com>, 
	Hugh Dickins <hughd@...gle.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>, 
	"Huang, Ying" <ying.huang@...ux.alibaba.com>, Kemeng Shi <shikemeng@...weicloud.com>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	"Matthew Wilcox (Oracle)" <willy@...radead.org>, linux-kernel@...r.kernel.org, 
	Kairui Song <kasong@...cent.com>
Subject: Re: [PATCH 16/19] mm, swap: check swap table directly for checking cache

On Thu, Oct 30, 2025 at 12:00 AM Kairui Song <ryncsn@...il.com> wrote:
>
> From: Kairui Song <kasong@...cent.com>
>
> Instead of looking at the swap map, check swap table directly to tell
> if a swap slot is cached. Prepares for the removal of SWAP_HAS_CACHE.
>
> Signed-off-by: Kairui Song <kasong@...cent.com>
> ---
>  mm/swap.h        | 11 ++++++++---
>  mm/swap_state.c  | 16 ++++++++++++++++
>  mm/swapfile.c    | 55 +++++++++++++++++++++++++++++--------------------------
>  mm/userfaultfd.c | 10 +++-------
>  4 files changed, 56 insertions(+), 36 deletions(-)
>
> diff --git a/mm/swap.h b/mm/swap.h
> index 03694ffa662f..73f07bcea5f0 100644
> --- a/mm/swap.h
> +++ b/mm/swap.h
> @@ -275,6 +275,7 @@ void __swapcache_clear_cached(struct swap_info_struct *si,
>   *   swap entries in the page table, similar to locking swap cache folio.
>   * - See the comment of get_swap_device() for more complex usage.
>   */
> +bool swap_cache_check_folio(swp_entry_t entry);
>  struct folio *swap_cache_get_folio(swp_entry_t entry);
>  void *swap_cache_get_shadow(swp_entry_t entry);
>  void swap_cache_del_folio(struct folio *folio);
> @@ -335,8 +336,6 @@ static inline int swap_zeromap_batch(swp_entry_t entry, int max_nr,
>
>  static inline int non_swapcache_batch(swp_entry_t entry, int max_nr)
>  {
> -       struct swap_info_struct *si = __swap_entry_to_info(entry);
> -       pgoff_t offset = swp_offset(entry);
>         int i;
>
>         /*
> @@ -345,8 +344,9 @@ static inline int non_swapcache_batch(swp_entry_t entry, int max_nr)
>          * be in conflict with the folio in swap cache.
>          */
>         for (i = 0; i < max_nr; i++) {
> -               if ((si->swap_map[offset + i] & SWAP_HAS_CACHE))
> +               if (swap_cache_check_folio(entry))
>                         return i;
> +               entry.val++;
>         }
>
>         return i;
> @@ -449,6 +449,11 @@ static inline int swap_writeout(struct folio *folio,
>         return 0;
>  }
>
> +static inline bool swap_cache_check_folio(swp_entry_t entry)
> +{
> +       return false;
> +}
> +
>  static inline struct folio *swap_cache_get_folio(swp_entry_t entry)
>  {
>         return NULL;
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 85d9f99c384f..41d4fa056203 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -103,6 +103,22 @@ struct folio *swap_cache_get_folio(swp_entry_t entry)
>         return NULL;
>  }
>
> +/**
> + * swap_cache_check_folio - Check if a swap slot has cache.
> + * @entry: swap entry indicating the slot.
> + *
> + * Context: Caller must ensure @entry is valid and protect the swap
> + * device with reference count or locks.
> + */
> +bool swap_cache_check_folio(swp_entry_t entry)
> +{
> +       unsigned long swp_tb;
> +
> +       swp_tb = swap_table_get(__swap_entry_to_cluster(entry),
> +                               swp_cluster_offset(entry));
> +       return swp_tb_is_folio(swp_tb);
> +}
> +

The name swap_cache_check_folio() sounds a bit odd to me — what we’re
actually doing is checking whether the swapcache contains (or is)
a folio, i.e., whether there’s a folio hit in the swapcache.
The word "check" could misleadingly suggest verifying the folio’s health
or validity instead.

what about swap_cache_has_folio() or simply:

struct folio *__swap_cache_get_folio(swp_entry_t entry);

This would return the folio without taking the lock, or NULL if not found?

Thanks
Barry