On Tue, Mar 18, 2025 at 2:10 PM Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> ---
> 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
>
