On Fri, Apr 24, 2026 at 12:00:58PM +0800, Li Wang <[email protected]> wrote: > 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/4 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 <[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]> > Acked-by: Nhat Pham <[email protected]> > --- > tools/testing/selftests/cgroup/test_zswap.c | 8 ++++---- > 1 file changed, 4 insertions(+), 4 deletions(-)
Reviewed-by: Michal Koutný <[email protected]>
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