Re: [PATCH] mm: swap: mTHP frees entries as a whole

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

On Tue, Aug 6, 2024 at 4:08 AM Zhiguo Jiang <justinjiang@vivo.com> wrote:
>
> Support mTHP's attempt to free swap entries as a whole, which can avoid
> frequent swap_info locking for every individual entry in
> swapcache_free_entries(). When the swap_map count values corresponding
> to all contiguous entries are all zero excluding SWAP_HAS_CACHE, the
> entries will be freed directly by skippping percpu swp_slots caches.
>

No, this isn't quite good. Please review the work done by Chris and Kairui[1];
they have handled it better. On a different note, I have a patch that can
handle zap_pte_range() for swap entries in batches[2][3].

[1] https://lore.kernel.org/linux-mm/20240730-swap-allocator-v5-5-cb9c148b9297@kernel.org/
[2] https://lore.kernel.org/linux-mm/20240803091118.84274-1-21cnbao@gmail.com/
[3] https://lore.kernel.org/linux-mm/CAGsJ_4wPnQqKOHx6iQcwO8bQzoBXKr2qY2AgSxMwTQCj3-8YWw@mail.gmail.com/

> Signed-off-by: Zhiguo Jiang <justinjiang@vivo.com>
> ---
>  mm/swapfile.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 61 insertions(+)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index ea023fc25d08..829fb4cfb6ec
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1493,6 +1493,58 @@ static void swap_entry_range_free(struct swap_info_struct *p, swp_entry_t entry,
>         swap_range_free(p, offset, nr_pages);
>  }
>
> +/*
> + * Free the contiguous swap entries as a whole, caller have to
> + * ensure all entries belong to the same folio.
> + */
> +static void swap_entry_range_check_and_free(struct swap_info_struct *p,
> +                                 swp_entry_t entry, int nr, bool *any_only_cache)
> +{
> +       const unsigned long start_offset = swp_offset(entry);
> +       const unsigned long end_offset = start_offset + nr;
> +       unsigned long offset;
> +       DECLARE_BITMAP(to_free, SWAPFILE_CLUSTER) = { 0 };
> +       struct swap_cluster_info *ci;
> +       int i = 0, nr_setbits = 0;
> +       unsigned char count;
> +
> +       /*
> +        * Free and check swap_map count values corresponding to all contiguous
> +        * entries in the whole folio range.
> +        */
> +       WARN_ON_ONCE(nr > SWAPFILE_CLUSTER);
> +       ci = lock_cluster_or_swap_info(p, start_offset);
> +       for (offset = start_offset; offset < end_offset; offset++, i++) {
> +               if (data_race(p->swap_map[offset])) {
> +                       count = __swap_entry_free_locked(p, offset, 1);
> +                       if (!count) {
> +                               bitmap_set(to_free, i, 1);
> +                               nr_setbits++;
> +                       } else if (count == SWAP_HAS_CACHE) {
> +                               *any_only_cache = true;
> +                       }
> +               } else {
> +                       WARN_ON_ONCE(1);
> +               }
> +       }
> +       unlock_cluster_or_swap_info(p, ci);
> +
> +       /*
> +        * If the swap_map count values corresponding to all contiguous entries are
> +        * all zero excluding SWAP_HAS_CACHE, the entries will be freed directly by
> +        * skippping percpu swp_slots caches, which can avoid frequent swap_info
> +        * locking for every individual entry.
> +        */
> +       if (nr > 1 && nr_setbits == nr) {
> +               spin_lock(&p->lock);
> +               swap_entry_range_free(p, entry, nr);
> +               spin_unlock(&p->lock);
> +       } else {
> +               for_each_set_bit(i, to_free, SWAPFILE_CLUSTER)
> +                       free_swap_slot(swp_entry(p->type, start_offset + i));
> +       }
> +}
> +
>  static void cluster_swap_free_nr(struct swap_info_struct *sis,
>                 unsigned long offset, int nr_pages,
>                 unsigned char usage)
> @@ -1808,6 +1860,14 @@ void free_swap_and_cache_nr(swp_entry_t entry, int nr)
>         if (WARN_ON(end_offset > si->max))
>                 goto out;
>
> +       /*
> +        * Try to free all contiguous entries about mTHP as a whole.
> +        */
> +       if (IS_ENABLED(CONFIG_THP_SWAP) && nr > 1) {
> +               swap_entry_range_check_and_free(si, entry, nr, &any_only_cache);
> +               goto free_cache;
> +       }
> +
>         /*
>          * First free all entries in the range.
>          */
> @@ -1821,6 +1881,7 @@ void free_swap_and_cache_nr(swp_entry_t entry, int nr)
>                 }
>         }
>
> +free_cache:
>         /*
>          * Short-circuit the below loop if none of the entries had their
>          * reference drop to zero.
> --
> 2.39.0
>