[PATCH v5 7/8] selftest/cgroup: fix zswap attempt_writeback() on 64K pagesize system

[Li Wang <liwang@redhat.com>]

In attempt_writeback(), a memsize of 4M only covers 64 pages on 64K
page size systems. When memory.reclaim is called, the kernel prefers
reclaiming clean file pages (binary, libc, linker, etc.) over swapping
anonymous pages. With only 64 pages of anonymous memory, the reclaim
target can be largely or entirely satisfied by dropping file pages,
resulting in very few or zero anonymous pages being pushed into zswap.

This causes zswap_usage to be extremely small or zero, making
zswap_usage/2 insufficient to create meaningful writeback pressure.
The test then fails because no writeback is triggered.

On 4K page size systems this is not an issue because 4M covers 1024
pages, and file pages are a small fraction of the reclaim target.

Fix this by:
- Always allocating 1024 pages regardless of page size. This ensures
  enough anonymous pages to reliably populate zswap and trigger
  writeback, while keeping the original 4M allocation on 4K systems.
- Setting zswap.max to zswap_usage/4 instead of zswap_usage/2 to
  create stronger writeback pressure, ensuring reclaim reliably
  triggers writeback even on large page size systems.

=== Error Log ===
  # uname -rm
  6.12.0-211.el10.ppc64le ppc64le

  # getconf PAGESIZE
  65536

  # ./test_zswap
  TAP version 13
  1..7
  ok 1 test_zswap_usage
  ok 2 test_swapin_nozswap
  ok 3 test_zswapin
  not ok 4 test_zswap_writeback_enabled
  ...

Signed-off-by: Li Wang <liwang@redhat.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: Nhat Pham <nphamcs@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Yosry Ahmed <yosry@kernel.org>
---

Notes:
    v5:
    	- Setting zswap.max to zswap_usage/4 to create stronger writeback pressure.
    v3-4:
    	- No changes.
    v2:
    	- use pagesize * 1024 which clearly shows the page numbers.

 tools/testing/selftests/cgroup/test_zswap.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
index 72082e5d4725..e96e316f7c47 100644
--- a/tools/testing/selftests/cgroup/test_zswap.c
+++ b/tools/testing/selftests/cgroup/test_zswap.c
@@ -273,7 +273,7 @@ static int test_zswapin(const char *root)
  */
 static int attempt_writeback(const char *cgroup, void *arg)
 {
-	size_t memsize = MB(4);
+	size_t memsize = page_size * 1024;
 	char buf[page_size];
 	long zswap_usage;
 	bool wb_enabled = *(bool *) arg;
@@ -311,7 +311,7 @@ static int attempt_writeback(const char *cgroup, void *arg)
 		}
 	}
 
-	if (cg_write_numeric(cgroup, "memory.zswap.max", zswap_usage/2))
+	if (cg_write_numeric(cgroup, "memory.zswap.max", zswap_usage/4))
 		goto out;
 
 	/*
-- 
2.53.0