mm, swap: swap entry of a bad slot should not be considered as swapped out

When checking if a swap entry is swapped out, we simply check if the
bitwise result of the count value is larger than 0.  But SWAP_MAP_BAD will
also be considered as a swao count value larger than 0.

SWAP_MAP_BAD being considered as a count value larger than 0 is useful for
the swap allocator: they will be seen as a used slot, so the allocator
will skip them.  But for the swapped out check, this isn't correct.

There is currently no observable issue.  The swapped out check is only
useful for readahead and folio swapped-out status check.  For readahead,
the swap cache layer will abort upon checking and updating the swap map.
For the folio swapped out status check, the swap allocator will never
allocate an entry of bad slots to folio, so that part is fine too.  The
worst that could happen now is redundant allocation/freeing of folios and
waste CPU time.

This also makes it easier to get rid of swap map checking and update
during folio insertion in the swap cache layer.

Link: https://lkml.kernel.org/r/20251220-swap-table-p2-v5-9-8862a265a033@tencent.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Rafael J. Wysocki (Intel) <rafael@kernel.org>
Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
Cc: Deepanshu Kartikey <kartikey406@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <ryncsn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/swap_state.c b/mm/swap_state.c
index a82d0615f64664c5537360f0a4ba24eebe349583..d58bce532d9599e6f0f4f21965e59cca94da5b7d 100644 (file)
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -526,7 +526,7 @@ struct folio *swap_cache_alloc_folio(swp_entry_t entry, gfp_t gfp_mask,
        if (folio)
                return folio;
 
-       /* Skip allocation for unused swap slot for readahead path. */
+       /* Skip allocation for unused and bad swap slot for readahead. */
        if (!swap_entry_swapped(si, entry))
                return NULL;
 

diff --git a/mm/swapfile.c b/mm/swapfile.c
index eb394f30181a525f15a1f97f0d8687fb0415085c..e47807fdf51b511f3b8bad6d2ac14517493b8620 100644 (file)
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1766,10 +1766,10 @@ int __swap_count(swp_entry_t entry)
        return swap_count(si->swap_map[offset]);
 }
 
-/*
- * How many references to @entry are currently swapped out?
- * This does not give an exact answer when swap count is continued,
- * but does include the high COUNT_CONTINUED flag to allow for that.
+/**
+ * swap_entry_swapped - Check if the swap entry is swapped.
+ * @si: the swap device.
+ * @entry: the swap entry.
  */
 bool swap_entry_swapped(struct swap_info_struct *si, swp_entry_t entry)
 {
@@ -1780,7 +1780,8 @@ bool swap_entry_swapped(struct swap_info_struct *si, swp_entry_t entry)
        ci = swap_cluster_lock(si, offset);
        count = swap_count(si->swap_map[offset]);
        swap_cluster_unlock(ci);
-       return !!count;
+
+       return count && count != SWAP_MAP_BAD;
 }
 
 /*
@@ -3677,10 +3678,10 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage, int nr)
                count = si->swap_map[offset + i];
 
                /*
-                * 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.
+                * For swapin out, allocator never allocates bad slots. for
+                * swapin, readahead is guarded by swap_entry_swapped.
                 */
-               if (unlikely(swap_count(count) == SWAP_MAP_BAD)) {
+               if (WARN_ON(swap_count(count) == SWAP_MAP_BAD)) {
                        err = -ENOENT;
                        goto unlock_out;
                }