[PATCH mm-unstable] mm/zswap: optimize and cleanup the invalidation of duplicate entry

[chengming.zhou@linux.dev]

From: Chengming Zhou <zhouchengming@bytedance.com>

We may encounter duplicate entry in the zswap_store():

1. swap slot that freed to per-cpu swap cache, doesn't invalidate
   the zswap entry, then got reused. This has been fixed.

2. !exclusive load mode, swapin folio will leave its zswap entry
   on the tree, then swapout again. This has been removed.

3. one folio can be dirtied again after zswap_store(), so need to
   zswap_store() again. This should be handled correctly.

So we must invalidate the old duplicate entry before inserting the
new one, which actually doesn't have to be done at the beginning
of zswap_store().

The good point is that we don't need to lock the tree twice in the
normal store success path. And cleanup the loop as we are here.

Note we still need to invalidate the old duplicate entry when store
failed or zswap is disabled , otherwise the new data in swapfile
could be overwrite by the old data in zswap pool when lru writeback.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Yosry Ahmed <yosryahmed@google.com>
Acked-by: Chris Li <chrisl@kernel.org>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
---
 mm/zswap.c | 34 ++++++++++++++++------------------
 1 file changed, 16 insertions(+), 18 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 96664cdee207..62fe307521c9 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1517,19 +1517,8 @@ bool zswap_store(struct folio *folio)
 	if (folio_test_large(folio))
 		return false;
 
-	/*
-	 * If this is a duplicate, it must be removed before attempting to store
-	 * it, otherwise, if the store fails the old page won't be removed from
-	 * the tree, and it might be written back overriding the new data.
-	 */
-	spin_lock(&tree->lock);
-	entry = zswap_rb_search(&tree->rbroot, offset);
-	if (entry)
-		zswap_invalidate_entry(tree, entry);
-	spin_unlock(&tree->lock);
-
 	if (!zswap_enabled)
-		return false;
+		goto check_old;
 
 	objcg = get_obj_cgroup_from_folio(folio);
 	if (objcg && !obj_cgroup_may_zswap(objcg)) {
@@ -1609,14 +1598,12 @@ bool zswap_store(struct folio *folio)
 	/* map */
 	spin_lock(&tree->lock);
 	/*
-	 * A duplicate entry should have been removed at the beginning of this
-	 * function. Since the swap entry should be pinned, if a duplicate is
-	 * found again here it means that something went wrong in the swap
-	 * cache.
+	 * The folio may have been dirtied again, invalidate the
+	 * possibly stale entry before inserting the new entry.
 	 */
-	while (zswap_rb_insert(&tree->rbroot, entry, &dupentry) == -EEXIST) {
-		WARN_ON(1);
+	if (zswap_rb_insert(&tree->rbroot, entry, &dupentry) == -EEXIST) {
 		zswap_invalidate_entry(tree, dupentry);
+		WARN_ON(zswap_rb_insert(&tree->rbroot, entry, &dupentry));
 	}
 	if (entry->length) {
 		INIT_LIST_HEAD(&entry->lru);
@@ -1639,6 +1626,17 @@ bool zswap_store(struct folio *folio)
 reject:
 	if (objcg)
 		obj_cgroup_put(objcg);
+check_old:
+	/*
+	 * If the zswap store fails or zswap is disabled, we must invalidate the
+	 * possibly stale entry which was previously stored at this offset.
+	 * Otherwise, writeback could overwrite the new data in the swapfile.
+	 */
+	spin_lock(&tree->lock);
+	entry = zswap_rb_search(&tree->rbroot, offset);
+	if (entry)
+		zswap_invalidate_entry(tree, entry);
+	spin_unlock(&tree->lock);
 	return false;
 
 shrink:
-- 
2.40.1