[PATCH v3 09/19] mm, swap: swap entry of a bad slot should not be considered as swapped out

[Kairui Song <ryncsn@gmail.com>]

From: Kairui Song <kasong@tencent.com>

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.

Signed-off-by: Kairui Song <kasong@tencent.com>
---
 include/linux/swap.h |  6 ++++--
 mm/swap_state.c      |  4 ++--
 mm/swapfile.c        | 22 +++++++++++-----------
 3 files changed, 17 insertions(+), 15 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index bf72b548a96d..936fa8f9e5f3 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -466,7 +466,8 @@ int find_first_swap(dev_t *device);
 extern unsigned int count_swap_pages(int, int);
 extern sector_t swapdev_block(int, pgoff_t);
 extern int __swap_count(swp_entry_t entry);
-extern bool swap_entry_swapped(struct swap_info_struct *si, swp_entry_t entry);
+extern bool swap_entry_swapped(struct swap_info_struct *si,
+			       unsigned long offset);
 extern int swp_swapcount(swp_entry_t entry);
 struct backing_dev_info;
 extern struct swap_info_struct *get_swap_device(swp_entry_t entry);
@@ -535,7 +536,8 @@ static inline int __swap_count(swp_entry_t entry)
 	return 0;
 }
 
-static inline bool swap_entry_swapped(struct swap_info_struct *si, swp_entry_t entry)
+static inline bool swap_entry_swapped(struct swap_info_struct *si,
+				      unsigned long offset)
 {
 	return false;
 }
diff --git a/mm/swap_state.c b/mm/swap_state.c
index e3c01e5bc978..a99411b70c99 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -527,8 +527,8 @@ 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. */
-	if (!swap_entry_swapped(si, entry))
+	/* Skip allocation for unused and bad swap slot for readahead. */
+	if (!swap_entry_swapped(si, swp_offset(entry)))
 		return NULL;
 
 	/* Allocate a new folio to be added into the swap cache. */
diff --git a/mm/swapfile.c b/mm/swapfile.c
index cc07246985ef..cb59930b6415 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1762,21 +1762,21 @@ 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 at @offset is swapped.
+ * @si: the swap device.
+ * @offset: offset of the swap entry.
  */
-bool swap_entry_swapped(struct swap_info_struct *si, swp_entry_t entry)
+bool swap_entry_swapped(struct swap_info_struct *si, unsigned long offset)
 {
-	pgoff_t offset = swp_offset(entry);
 	struct swap_cluster_info *ci;
 	int count;
 
 	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;
 }
 
 /*
@@ -1862,7 +1862,7 @@ static bool folio_swapped(struct folio *folio)
 		return false;
 
 	if (!IS_ENABLED(CONFIG_THP_SWAP) || likely(!folio_test_large(folio)))
-		return swap_entry_swapped(si, entry);
+		return swap_entry_swapped(si, swp_offset(entry));
 
 	return swap_page_trans_huge_swapped(si, entry, folio_order(folio));
 }
@@ -3673,10 +3673,10 @@ 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.
+		 * Allocator never allocates bad slots, and 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;
 		}

-- 
2.52.0