Re: [PATCH RFC v2 4/5] mm: swap: introduce swapcache_prepare_nr and swapcache_clear_nr for large folios swap-in

[Yosry Ahmed]

On Wed, Feb 28, 2024 at 4:39 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> From: Barry Song <v-songbaohua@xxxxxxxx>
>
> Commit 13ddaf26be32 ("mm/swap: fix race when skipping swapcache") supports
> one entry only, to support large folio swap-in, we need to handle multiple
> swap entries.
>
> Cc: Kairui Song <kasong@xxxxxxxxxxx>
> Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Chris Li <chrisl@xxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
> Cc: SeongJae Park <sj@xxxxxxxxxx>
> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> ---
>  include/linux/swap.h |   1 +
>  mm/swap.h            |   1 +
>  mm/swapfile.c        | 117 ++++++++++++++++++++++++++-----------------
>  3 files changed, 72 insertions(+), 47 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index b3581c976e5f..2691c739d9a4 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -480,6 +480,7 @@ extern int add_swap_count_continuation(swp_entry_t, gfp_t);
>  extern void swap_shmem_alloc(swp_entry_t);
>  extern int swap_duplicate(swp_entry_t);
>  extern int swapcache_prepare(swp_entry_t);
> +extern int swapcache_prepare_nr(swp_entry_t, int nr);
>  extern void swap_free(swp_entry_t);
>  extern void swap_nr_free(swp_entry_t entry, int nr_pages);
>  extern void swapcache_free_entries(swp_entry_t *entries, int n);
> diff --git a/mm/swap.h b/mm/swap.h
> index fc2f6ade7f80..1cec991efcda 100644
> --- a/mm/swap.h
> +++ b/mm/swap.h
> @@ -42,6 +42,7 @@ void delete_from_swap_cache(struct folio *folio);
>  void clear_shadow_from_swap_cache(int type, unsigned long begin,
>                                   unsigned long end);
>  void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry);
> +void swapcache_clear_nr(struct swap_info_struct *si, swp_entry_t entry, int nr);
>  struct folio *swap_cache_get_folio(swp_entry_t entry,
>                 struct vm_area_struct *vma, unsigned long addr);
>  struct folio *filemap_get_incore_folio(struct address_space *mapping,
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index c0c058ee7b69..c8c8b6dbaeda 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -3308,7 +3308,7 @@ void si_swapinfo(struct sysinfo *val)
>  }
>
>  /*
> - * Verify that a swap entry is valid and increment its swap map count.
> + * Verify that nr swap entries are valid and increment their swap map count.
>   *
>   * Returns error code in following case.
>   * - success -> 0
> @@ -3318,66 +3318,73 @@ void si_swapinfo(struct sysinfo *val)
>   * - swap-cache reference is requested but the entry is not used. -> ENOENT
>   * - swap-mapped reference requested but needs continued swap count. -> ENOMEM
>   */
> -static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
> +static int __swap_duplicate_nr(swp_entry_t entry, int nr, unsigned char usage)
>  {
>         struct swap_info_struct *p;
>         struct swap_cluster_info *ci;
>         unsigned long offset;
> -       unsigned char count;
> -       unsigned char has_cache;
> -       int err;
> +       unsigned char count[SWAPFILE_CLUSTER];
> +       unsigned char has_cache[SWAPFILE_CLUSTER];

I am not closely following this series, but a couple of things caught my eyes.

Is this reasonable for stack usage?

> +       int err, i;
>
>         p = swp_swap_info(entry);
>
>         offset = swp_offset(entry);
>         ci = lock_cluster_or_swap_info(p, offset);
>
> -       count = p->swap_map[offset];
> -
> -       /*
> -        * swapin_readahead() doesn't check if a swap entry is valid, so the
> -        * swap entry could be SWAP_MAP_BAD. Check here with lock held.
> -        */
> -       if (unlikely(swap_count(count) == SWAP_MAP_BAD)) {
> -               err = -ENOENT;
> -               goto unlock_out;
> -       }
> +       for (i = 0; i < nr; i++) {
> +               count[i] = p->swap_map[offset + i];
>
> -       has_cache = count & SWAP_HAS_CACHE;
> -       count &= ~SWAP_HAS_CACHE;
> -       err = 0;
> -
> -       if (usage == SWAP_HAS_CACHE) {
> -
> -               /* set SWAP_HAS_CACHE if there is no cache and entry is used */
> -               if (!has_cache && count)
> -                       has_cache = SWAP_HAS_CACHE;
> -               else if (has_cache)             /* someone else added cache */
> -                       err = -EEXIST;
> -               else                            /* no users remaining */
> +               /*
> +                * swapin_readahead() doesn't check if a swap entry is valid, so the
> +                * swap entry could be SWAP_MAP_BAD. Check here with lock held.
> +                */
> +               if (unlikely(swap_count(count[i]) == SWAP_MAP_BAD)) {
>                         err = -ENOENT;
> +                       goto unlock_out;
> +               }

Here we immediately exit if there is an error, but we don't below, we
just keep overwriting the error every iteration as far as I can tell.
Also, it doesn't seem like we do any cleanups if we hit an error
halfway through. Should we undo previously updated swap entries, or am
I missing something here?

>
> -       } else if (count || has_cache) {
> -
> -               if ((count & ~COUNT_CONTINUED) < SWAP_MAP_MAX)
> -                       count += usage;
> -               else if ((count & ~COUNT_CONTINUED) > SWAP_MAP_MAX)
> -                       err = -EINVAL;
> -               else if (swap_count_continued(p, offset, count))
> -                       count = COUNT_CONTINUED;
> -               else
> -                       err = -ENOMEM;
> -       } else
> -               err = -ENOENT;                  /* unused swap entry */
> -
> -       if (!err)
> -               WRITE_ONCE(p->swap_map[offset], count | has_cache);
> +               has_cache[i] = count[i] & SWAP_HAS_CACHE;
> +               count[i] &= ~SWAP_HAS_CACHE;
> +               err = 0;
> +
> +               if (usage == SWAP_HAS_CACHE) {
> +
> +                       /* set SWAP_HAS_CACHE if there is no cache and entry is used */
> +                       if (!has_cache[i] && count[i])
> +                               has_cache[i] = SWAP_HAS_CACHE;
> +                       else if (has_cache[i])          /* someone else added cache */
> +                               err = -EEXIST;
> +                       else                            /* no users remaining */
> +                               err = -ENOENT;
> +               } else if (count[i] || has_cache[i]) {
> +
> +                       if ((count[i] & ~COUNT_CONTINUED) < SWAP_MAP_MAX)
> +                               count[i] += usage;
> +                       else if ((count[i] & ~COUNT_CONTINUED) > SWAP_MAP_MAX)
> +                               err = -EINVAL;
> +                       else if (swap_count_continued(p, offset + i, count[i]))
> +                               count[i] = COUNT_CONTINUED;
> +                       else
> +                               err = -ENOMEM;
> +               } else
> +                       err = -ENOENT;                  /* unused swap entry */
> +       }
>
> +       if (!err) {
> +               for (i = 0; i < nr; i++)
> +                       WRITE_ONCE(p->swap_map[offset + i], count[i] | has_cache[i]);
> +       }
>  unlock_out:
>         unlock_cluster_or_swap_info(p, ci);
>         return err;
>  }