[PATCH v4 5/9] mm/shmem, swap: avoid false positive swap cache lookup

[Kairui Song <ryncsn@gmail.com>]

From: Kairui Song <kasong@tencent.com>

If a shmem read request's index points to the middle of a large swap
entry, shmem swap in will try the swap cache lookup using the large
swap entry's starting value (which is the first sub swap entry of this
large entry).  This will lead to false positive lookup results, if only
the first few swap entries are cached but the actual requested swap
entry pointed by index is uncached. This is not a rare event as swap
readahead always try to cache order 0 folios when possible.

Currently, shmem will do a large entry split when it occurs, aborts
due to a mismatching folio swap value, then retry the swapin from
the beginning, which is a waste of CPU and adds wrong info to
the readahead statistics.

This can be optimized easily by doing the lookup using the right
swap entry value.

Signed-off-by: Kairui Song <kasong@tencent.com>
---
 mm/shmem.c | 31 +++++++++++++++----------------
 1 file changed, 15 insertions(+), 16 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 217264315842..2ab214e2771c 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2274,14 +2274,15 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 	pgoff_t offset;
 
 	VM_BUG_ON(!*foliop || !xa_is_value(*foliop));
-	swap = index_entry = radix_to_swp_entry(*foliop);
+	index_entry = radix_to_swp_entry(*foliop);
+	swap = index_entry;
 	*foliop = NULL;
 
-	if (is_poisoned_swp_entry(swap))
+	if (is_poisoned_swp_entry(index_entry))
 		return -EIO;
 
-	si = get_swap_device(swap);
-	order = shmem_confirm_swap(mapping, index, swap);
+	si = get_swap_device(index_entry);
+	order = shmem_confirm_swap(mapping, index, index_entry);
 	if (unlikely(!si)) {
 		if (order < 0)
 			return -EEXIST;
@@ -2293,6 +2294,12 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 		return -EEXIST;
 	}
 
+	/* index may point to the middle of a large entry, get the sub entry */
+	if (order) {
+		offset = index - round_down(index, 1 << order);
+		swap = swp_entry(swp_type(swap), swp_offset(swap) + offset);
+	}
+
 	/* Look it up and read it in.. */
 	folio = swap_cache_get_folio(swap, NULL, 0);
 	if (!folio) {
@@ -2305,8 +2312,10 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 
 		/* Skip swapcache for synchronous device. */
 		if (data_race(si->flags & SWP_SYNCHRONOUS_IO)) {
-			folio = shmem_swap_alloc_folio(inode, vma, index, swap, order, gfp);
+			folio = shmem_swap_alloc_folio(inode, vma, index,
+						       index_entry, order, gfp);
 			if (!IS_ERR(folio)) {
+				swap = index_entry;
 				skip_swapcache = true;
 				goto alloced;
 			}
@@ -2320,17 +2329,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 			if (error == -EEXIST)
 				goto failed;
 		}
-
-		/*
-		 * Now swap device can only swap in order 0 folio, it is
-		 * necessary to recalculate the new swap entry based on
-		 * the offset, as the swapin index might be unalgined.
-		 */
-		if (order) {
-			offset = index - round_down(index, 1 << order);
-			swap = swp_entry(swp_type(swap), swp_offset(swap) + offset);
-		}
-
+		/* Cached swapin with readahead, only supports order 0 */
 		folio = shmem_swapin_cluster(swap, gfp, info, index);
 		if (!folio) {
 			error = -ENOMEM;
-- 
2.50.0