+ selftests-cgroup-test_zswap-wait-for-asynchronous-writeback.patch added to mm-new branch

[Andrew Morton <akpm@linux-foundation.org>]

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The patch titled
     Subject: selftests/cgroup: test_zswap: wait for asynchronous writeback
has been added to the -mm mm-new branch.  Its filename is
     selftests-cgroup-test_zswap-wait-for-asynchronous-writeback.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/selftests-cgroup-test_zswap-wait-for-asynchronous-writeback.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

The mm-new branch of mm.git is not included in linux-next

If a few days of testing in mm-new is successful, the patch will me moved
into mm.git's mm-unstable branch, which is included in linux-next

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------------------------------------------------------
From: Li Wang <li.wang@linux.dev>
Subject: selftests/cgroup: test_zswap: wait for asynchronous writeback
Date: Fri, 24 Apr 2026 12:00:59 +0800

zswap writeback is asynchronous, but test_zswap.c checks writeback
counters immediately after reclaim/trigger paths.  On some platforms (e.g.
ppc64le), this can race with background writeback and cause spurious
failures even when behavior is correct.

Add wait_for_writeback() to poll get_cg_wb_count() with a bounded
timeout, and use it in:

  test_zswap_writeback_one() when writeback is expected
  test_no_invasive_cgroup_shrink() for the wb_group check

This keeps the original before/after assertion style while making the
tests robust against writeback completion latency.

No test behavior change, selftest stability improvement only.

Link: https://lore.kernel.org/20260424040059.12940-9-li.wang@linux.dev
Signed-off-by: Li Wang <li.wang@linux.dev>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Jiayuan Chen <jiayuan.chen@linux.dev>
Cc: Waiman Long <longman@redhat.com>
Cc: Yosry Ahmed <yosry@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 tools/testing/selftests/cgroup/test_zswap.c |   28 ++++++++++++++++--
 1 file changed, 26 insertions(+), 2 deletions(-)

--- a/tools/testing/selftests/cgroup/test_zswap.c~selftests-cgroup-test_zswap-wait-for-asynchronous-writeback
+++ a/tools/testing/selftests/cgroup/test_zswap.c
@@ -121,6 +121,27 @@ fail:
 }
 
 /*
+ * Writeback is asynchronous; poll until at least one writeback has
+ * been recorded for @cg, or until @timeout_ms has elapsed.
+ */
+static long wait_for_writeback(const char *cg, int timeout_ms)
+{
+	long elapsed, count;
+	for (elapsed = 0; elapsed < timeout_ms; elapsed += 100) {
+		count = get_cg_wb_count(cg);
+
+		if (count < 0)
+			return -1;
+		if (count > 0)
+			return count;
+
+		usleep(100000);
+	}
+
+	return 0;
+}
+
+/*
  * Sanity test to check that pages are written into zswap.
  */
 static int test_zswap_usage(const char *root)
@@ -345,7 +366,10 @@ static int test_zswap_writeback_one(cons
 		return -1;
 
 	/* Verify that zswap writeback occurred only if writeback was enabled */
-	zswpwb_after = get_cg_wb_count(cgroup);
+	if (wb)
+		zswpwb_after = wait_for_writeback(cgroup, 5000);
+	else
+		zswpwb_after = get_cg_wb_count(cgroup);
 	if (zswpwb_after < 0)
 		return -1;
 
@@ -476,7 +500,7 @@ static int test_no_invasive_cgroup_shrin
 	}
 
 	/* Verify that only zswapped memory from gwb_group has been written back */
-	if (get_cg_wb_count(wb_group) > 0 && get_cg_wb_count(zw_group) == 0)
+	if (wait_for_writeback(wb_group, 5000) > 0 && get_cg_wb_count(zw_group) == 0)
 		ret = KSFT_PASS;
 out:
 	cg_enter_current(root);
_

Patches currently in -mm which might be from li.wang@linux.dev are

maintainers-update-li-wangs-email-address.patch
selftests-cgroup-skip-test_zswap-if-zswap-is-globally-disabled.patch
selftests-cgroup-avoid-oom-in-test_swapin_nozswap.patch
selftests-cgroup-use-runtime-page-size-for-zswpin-check.patch
selftests-cgroup-rename-page_size-to-buf_size-in-cgroup_util.patch
selftests-cgroup-replace-hardcoded-page-size-values-in-test_zswap.patch
selftest-cgroup-fix-zswap-test_no_invasive_cgroup_shrink-on-large-pagesize-system.patch
selftest-cgroup-fix-zswap-attempt_writeback-on-64k-pagesize-system.patch
selftests-cgroup-test_zswap-wait-for-asynchronous-writeback.patch