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 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 <[email protected]> Cc: Johannes Weiner <[email protected]> Cc: Michal Hocko <[email protected]> Cc: Michal Koutný <[email protected]> Cc: Muchun Song <[email protected]> Cc: Nhat Pham <[email protected]> Cc: Tejun Heo <[email protected]> Cc: Roman Gushchin <[email protected]> Cc: Shakeel Butt <[email protected]> Acked-by: Yosry Ahmed <[email protected]> --- Notes: v3: - No changes. v2: - use pagesize * 1024 which clearly shows the page numbers. tools/testing/selftests/cgroup/test_zswap.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c index f84e84c3e7c8..d7a862a414e5 100644 --- a/tools/testing/selftests/cgroup/test_zswap.c +++ b/tools/testing/selftests/cgroup/test_zswap.c @@ -269,7 +269,7 @@ static int test_zswapin(const char *root) */ static int attempt_writeback(const char *cgroup, void *arg) { - size_t memsize = MB(4); + size_t memsize = pagesize * 1024; char buf[pagesize]; long zswap_usage; bool wb_enabled = *(bool *) arg; -- 2.53.0
