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

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Message-ID: <CAMgjq7Dwhtt4ACmOFSqEvzSoA+Ny0D9_1yeZz9gf7bWvSjj9jA@mail.gmail.com>
Date: Fri, 7 Nov 2025 11:13:07 +0800
From: Kairui Song <ryncsn@...il.com>
To: Barry Song <21cnbao@...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
Subject: Re: [PATCH 16/19] mm, swap: check swap table directly for checking cache

On Fri, Nov 7, 2025 at 5:03 AM Barry Song <21cnbao@...il.com> wrote:
>
> 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);

I was worrying people may misuse this, the returned folio could be
invalided anytime if caller is not holding rcu lock.

I think swap_cache_has_folio seems better indeed.