[PATCH 5/9] mm/shmem, swap: remove redundant error handling for replacing folio

[Kairui Song <ryncsn@gmail.com>]

From: Kairui Song <kasong@tencent.com>

Shmem may replace a folio in the swap cache if the cached one doesn't
fit the swapin's GFP zone. When doing so, shmem has already double
checked that the swap cache folio is locked, still has the swap cache
flag set, and contains the wanted swap entry. So it is impossible to
fail due to an Xarray mismatch. There is even a comment for that.

Delete the defensive error handling path, and add a WARN_ON instead:
if that happened, something has broken the basic principle of how the
swap cache works, we should catch and fix that.

Signed-off-by: Kairui Song <kasong@tencent.com>
---
 mm/shmem.c | 28 +++-------------------------
 1 file changed, 3 insertions(+), 25 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index b4d39f2a1e0a..e03793cc5169 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2158,35 +2158,13 @@ static int shmem_replace_folio(struct folio **foliop, gfp_t gfp,
 	/* Swap cache still stores N entries instead of a high-order entry */
 	xa_lock_irq(&swap_mapping->i_pages);
 	for (i = 0; i < nr_pages; i++) {
-		void *item = xas_load(&xas);
-
-		if (item != old) {
-			error = -ENOENT;
-			break;
-		}
-
-		xas_store(&xas, new);
+		WARN_ON_ONCE(xas_store(&xas, new));
 		xas_next(&xas);
 	}
-	if (!error) {
-		mem_cgroup_replace_folio(old, new);
-		shmem_update_stats(new, nr_pages);
-		shmem_update_stats(old, -nr_pages);
-	}
 	xa_unlock_irq(&swap_mapping->i_pages);
 
-	if (unlikely(error)) {
-		/*
-		 * Is this possible?  I think not, now that our callers
-		 * check both the swapcache flag and folio->private
-		 * after getting the folio lock; but be defensive.
-		 * Reverse old to newpage for clear and free.
-		 */
-		old = new;
-	} else {
-		folio_add_lru(new);
-		*foliop = new;
-	}
+	folio_add_lru(new);
+	*foliop = new;
 
 	folio_clear_swapcache(old);
 	old->private = NULL;
-- 
2.51.0