Re: [PATCH v2 2/8] mm: swap: remove unneeded VM_BUG_ON(*map != SWAP_HAS_CACHE) in swap_entry_range_free()

[Kairui Song <ryncsn@gmail.com>]

On Tue, Mar 18, 2025 at 2:10 PM Kemeng Shi <shikemeng@huaweicloud.com> wrote:
>
> As all callers of swap_entry_range_free() have already ensured slots to
> be freed are marked as SWAP_HAS_CACHE while holding the cluster lock,
> the BUG_ON check can be safely removed. After this, the function
> swap_entry_range_free() could drop any kind of last flag, rename it to
> swap_entries_free() and update it's comment accordingly.
>
> This is a preparation to use swap_entries_free() to drop last ref count
> and SWAP_MAP_SHMEM flag.
>
> Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
> Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
> ---
>  mm/swapfile.c | 27 +++++++++++++--------------
>  1 file changed, 13 insertions(+), 14 deletions(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 5a775456e26c..0aa7ce82c013 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -52,9 +52,9 @@
>  static bool swap_count_continued(struct swap_info_struct *, pgoff_t,
>                                  unsigned char);
>  static void free_swap_count_continuations(struct swap_info_struct *);
> -static void swap_entry_range_free(struct swap_info_struct *si,
> -                                 struct swap_cluster_info *ci,
> -                                 swp_entry_t entry, unsigned int nr_pages);
> +static void swap_entries_free(struct swap_info_struct *si,
> +                             struct swap_cluster_info *ci,
> +                             swp_entry_t entry, unsigned int nr_pages);
>  static void swap_range_alloc(struct swap_info_struct *si,
>                              unsigned int nr_entries);
>  static bool folio_swapcache_freeable(struct folio *folio);
> @@ -1463,7 +1463,7 @@ static unsigned char swap_entry_put(struct swap_info_struct *si,
>         ci = lock_cluster(si, offset);
>         usage = swap_entry_put_locked(si, offset, 1);
>         if (!usage)
> -               swap_entry_range_free(si, ci, swp_entry(si->type, offset), 1);
> +               swap_entries_free(si, ci, swp_entry(si->type, offset), 1);
>         unlock_cluster(ci);
>
>         return usage;
> @@ -1493,7 +1493,7 @@ static bool swap_entries_put_nr(struct swap_info_struct *si,
>         for (i = 0; i < nr; i++)
>                 WRITE_ONCE(si->swap_map[offset + i], SWAP_HAS_CACHE);
>         if (!has_cache)
> -               swap_entry_range_free(si, ci, entry, nr);
> +               swap_entries_free(si, ci, entry, nr);
>         unlock_cluster(ci);
>
>         return has_cache;
> @@ -1512,12 +1512,12 @@ static bool swap_entries_put_nr(struct swap_info_struct *si,
>  }
>
>  /*
> - * Drop the last HAS_CACHE flag of swap entries, caller have to
> - * ensure all entries belong to the same cgroup.
> + * Drop the last flag(1, SWAP_HAS_CACHE or SWAP_MAP_SHMEM) of swap entries,
> + * caller have to ensure all entries belong to the same cgroup and cluster.
>   */
> -static void swap_entry_range_free(struct swap_info_struct *si,
> -                                 struct swap_cluster_info *ci,
> -                                 swp_entry_t entry, unsigned int nr_pages)
> +static void swap_entries_free(struct swap_info_struct *si,
> +                             struct swap_cluster_info *ci,
> +                             swp_entry_t entry, unsigned int nr_pages)
>  {
>         unsigned long offset = swp_offset(entry);
>         unsigned char *map = si->swap_map + offset;
> @@ -1530,7 +1530,6 @@ static void swap_entry_range_free(struct swap_info_struct *si,
>
>         ci->count -= nr_pages;
>         do {
> -               VM_BUG_ON(*map != SWAP_HAS_CACHE);

Hi Kemeng

Instead of just dropping this check, maybe it's better to change it to
something like VM_BUG_ON(!*map); to catch potential SWAP double free?
I've found this check very helpful for debugging, especially for
catching concurrency problems.

Or more strictly: VM_BUG_ON(*map !=  SWAP_HAS_CACHE && *map != 1 &&
*map != SWAP_MAP_SHMEM);, you may introduce a helper to check if a
entry is the "last map" like this and use it somewhere else too.


>                 *map = 0;
>         } while (++map < map_end);
>
> @@ -1553,7 +1552,7 @@ static void cluster_swap_free_nr(struct swap_info_struct *si,
>         ci = lock_cluster(si, offset);
>         do {
>                 if (!swap_entry_put_locked(si, offset, usage))
> -                       swap_entry_range_free(si, ci, swp_entry(si->type, offset), 1);
> +                       swap_entries_free(si, ci, swp_entry(si->type, offset), 1);
>         } while (++offset < end);
>         unlock_cluster(ci);
>  }
> @@ -1596,11 +1595,11 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry)
>
>         ci = lock_cluster(si, offset);
>         if (swap_only_has_cache(si, offset, size))
> -               swap_entry_range_free(si, ci, entry, size);
> +               swap_entries_free(si, ci, entry, size);
>         else {
>                 for (int i = 0; i < size; i++, entry.val++) {
>                         if (!swap_entry_put_locked(si, offset + i, SWAP_HAS_CACHE))
> -                               swap_entry_range_free(si, ci, entry, 1);
> +                               swap_entries_free(si, ci, entry, 1);
>                 }
>         }
>         unlock_cluster(ci);
> --
> 2.30.0
>