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mm-zswap-fix-global-shrinker-error-handling-logic.patch removed from -mm tree

merged mm-stable Andrew Morton 1 participant 1 message Sep 2, 2024

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The quilt patch titled
Subject: mm: zswap: fix global shrinker error handling logic
has been removed from the -mm tree. Its filename was
mm-zswap-fix-global-shrinker-error-handling-logic.patch

This patch was dropped because it was merged into the mm-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: Takero Funaki <[email protected]>
Subject: mm: zswap: fix global shrinker error handling logic
Date: Wed, 31 Jul 2024 00:49:10 +0000

This patch fixes the zswap global shrinker, which did not shrink the zpool as expected.

The issue addressed is that shrink_worker() did not distinguish between unexpected errors and expected errors, such as failed writeback from an empty memcg. The shrinker would stop shrinking after iterating through the memcg tree 16 times, even if there was only one empty memcg.

With this patch, the shrinker no longer considers encountering an empty memcg, encountering a memcg with writeback disabled, or reaching the end of a memcg tree walk as a failure, as long as there are memcgs that are candidates for writeback. Systems with one or more empty memcgs will now observe significantly higher zswap writeback activity after the zswap pool limit is hit.

To avoid an infinite loop when there are no writeback candidates, this patch tracks writeback attempts during memcg tree walks and limits reties if no writeback candidates are found.

To handle the empty memcg case, the helper function shrink_memcg() is modified to check if the memcg is empty and then return -ENOENT.

Link https://lkml.kernel.org/r/[email protected] open here
Fixes a65b0e7607cc ("zswap: make shrinking memcg-aware")
Signed-off-by Takero Funaki <[email protected]>
Reviewed-by Chengming Zhou <[email protected]>
Reviewed-by Nhat Pham <[email protected]>
Cc Johannes Weiner <[email protected]>
Cc Yosry Ahmed <[email protected]>
Signed-off-by Andrew Morton <[email protected]>

---

 mm/zswap.c |   40 +++++++++++++++++++++++++++++++++-------
 1 file changed, 33 insertions(+), 7 deletions(-)
mm/zswap.c renamed from mm/zswap.c~mm-zswap-fix-global-shrinker-error-handling-logic +33 -7
@@ -1287,10 +1287,10 @@ static struct shrinker *zswap_alloc_shri
 
 static int shrink_memcg(struct mem_cgroup *memcg)
 {
-	int nid, shrunk = 0;
+	int nid, shrunk = 0, scanned = 0;
 
 	if (!mem_cgroup_zswap_writeback_enabled(memcg))
-		return -EINVAL;
+		return -ENOENT;
 
 	/*
 	 * Skip zombies because their LRUs are reparented and we would be

@@ -1304,21 +1304,34 @@ static int shrink_memcg(struct mem_cgrou
 
 		shrunk += list_lru_walk_one(&zswap_list_lru, nid, memcg,
 					    &shrink_memcg_cb, NULL, &nr_to_walk);
+		scanned += 1 - nr_to_walk;
 	}
+
+	if (!scanned)
+		return -ENOENT;
+
 	return shrunk ? 0 : -EAGAIN;
 }
 
 static void shrink_worker(struct work_struct *w)
 {
 	struct mem_cgroup *memcg;
-	int ret, failures = 0;
+	int ret, failures = 0, attempts = 0;
 	unsigned long thr;
 
 	/* Reclaim down to the accept threshold */
 	thr = zswap_accept_thr_pages();
 
 	/*
-	 * Global reclaim will select cgroup in a round-robin fashion.
+	 * Global reclaim will select cgroup in a round-robin fashion from all
+	 * online memcgs, but memcgs that have no pages in zswap and
+	 * writeback-disabled memcgs (memory.zswap.writeback=0) are not
+	 * candidates for shrinking.
+	 *
+	 * Shrinking will be aborted if we encounter the following
+	 * MAX_RECLAIM_RETRIES times:
+	 * - No writeback-candidate memcgs found in a memcg tree walk.
+	 * - Shrinking a writeback-candidate memcg failed.
 	 *
 	 * We save iteration cursor memcg into zswap_next_shrink,
 	 * which can be modified by the offline memcg cleaner

@@ -1354,9 +1367,14 @@ static void shrink_worker(struct work_st
 		spin_unlock(&zswap_shrink_lock);
 
 		if (!memcg) {
-			if (++failures == MAX_RECLAIM_RETRIES)
+			/*
+			 * Continue shrinking without incrementing failures if
+			 * we found candidate memcgs in the last tree walk.
+			 */
+			if (!attempts && ++failures == MAX_RECLAIM_RETRIES)
 				break;
 
+			attempts = 0;
 			goto resched;
 		}
 

@@ -1364,8 +1382,16 @@ static void shrink_worker(struct work_st
 		/* drop the extra reference */
 		mem_cgroup_put(memcg);
 
-		if (ret == -EINVAL)
-			break;
+		/*
+		 * There are no writeback-candidate pages in the memcg.
+		 * This is not an issue as long as we can find another memcg
+		 * with pages in zswap. Skip this without incrementing attempts
+		 * and failures.
+		 */
+		if (ret == -ENOENT)
+			continue;
+		++attempts;
+
 		if (ret && ++failures == MAX_RECLAIM_RETRIES)
 			break;
 resched:

_

Patches currently in -mm which might be from [email protected] are