Re: [PATCH 5/5] selftest/cgroup: fix zswap attempt_writeback() on 64K pagesize system

[Li Wang <liwang@redhat.com>]

On Wed, Mar 11, 2026 at 11:58:34AM -0700, Yosry Ahmed wrote:
> On Wed, Mar 11, 2026 at 4:06 AM Li Wang <liwang@redhat.com> 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/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.
> >
> > Scale memsize up to 64M on systems with page size larger than 4K, so
> > that enough anonymous pages are allocated to reliably populate zswap
> > and trigger writeback. The original 4M is kept for 4K page size systems
> > to avoid unnecessary memory usage and test runtime.
> >
> > === 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>
> > Cc: Yosry Ahmed <yosryahmed@google.com>
> > ---
> >  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 30d3fbf6b4fb..2d065184eea4 100644
> > --- a/tools/testing/selftests/cgroup/test_zswap.c
> > +++ b/tools/testing/selftests/cgroup/test_zswap.c
> > @@ -264,7 +264,7 @@ static int test_zswapin(const char *root)
> >  static int attempt_writeback(const char *cgroup, void *arg)
> >  {
> >         long pagesize = sysconf(_SC_PAGESIZE);
> > -       size_t memsize = MB(4);
> > +       size_t memsize = pagesize > 4096 ? MB(64): MB(4);
> 
> pagesize << 10 or pagesize * 1024?

Sure, I will go pagesize * 1024 which clearly shows the numbers of page.

-- 
Regards,
Li Wang