selftests/cgroup: avoid OOM in test_swapin_nozswap

test_swapin_nozswap can hit OOM before reaching its assertions on some
setups.  The test currently sets memory.max=8M and then allocates/reads
32M with memory.zswap.max=0, which may over-constrain reclaim and kill the
workload process.

Replace hardcoded sizes with PAGE_SIZE-based values:
  - control_allocation_size = PAGE_SIZE * 512
  - memory.max = control_allocation_size * 3 / 4
  - minimum expected swap = control_allocation_size / 4

This keeps the test pressure model intact (allocate/read beyond memory.max
to force swap-in/out) while making it more robust across different
environments.

The test intent is unchanged: confirm that swapping occurs while zswap remains
unused when memory.zswap.max=0.

=== Error Logs ===

  # ./test_zswap
  TAP version 13
  1..7
  ok 1 test_zswap_usage
  not ok 2 test_swapin_nozswap
  ...

  # dmesg
  [271641.879153] test_zswap invoked oom-killer: gfp_mask=0xcc0(GFP_KERNEL), order=0, oom_score_adj=0
  [271641.879168] CPU: 1 UID: 0 PID: 177372 Comm: test_zswap Kdump: loaded Not tainted 6.12.0-211.el10.ppc64le #1 VOLUNTARY
  [271641.879171] Hardware name: IBM,9009-41A POWER9 (architected) 0x4e0202 0xf000005 of:IBM,FW940.02 (UL940_041) hv:phyp pSeries
  [271641.879173] Call Trace:
  [271641.879174] [c00000037540f730] [c00000000127ec44] dump_stack_lvl+0x88/0xc4 (unreliable)
  [271641.879184] [c00000037540f760] [c0000000005cc594] dump_header+0x5c/0x1e4
  [271641.879188] [c00000037540f7e0] [c0000000005cb464] oom_kill_process+0x324/0x3b0
  [271641.879192] [c00000037540f860] [c0000000005cbe48] out_of_memory+0x118/0x420
  [271641.879196] [c00000037540f8f0] [c00000000070d8ec] mem_cgroup_out_of_memory+0x18c/0x1b0
  [271641.879200] [c00000037540f990] [c000000000713888] try_charge_memcg+0x598/0x890
  [271641.879204] [c00000037540fa70] [c000000000713dbc] charge_memcg+0x5c/0x110
  [271641.879207] [c00000037540faa0] [c0000000007159f8] __mem_cgroup_charge+0x48/0x120
  [271641.879211] [c00000037540fae0] [c000000000641914] alloc_anon_folio+0x2b4/0x5a0
  [271641.879215] [c00000037540fb60] [c000000000641d58] do_anonymous_page+0x158/0x6b0
  [271641.879218] [c00000037540fbd0] [c000000000642f8c] __handle_mm_fault+0x4bc/0x910
  [271641.879221] [c00000037540fcf0] [c000000000643500] handle_mm_fault+0x120/0x3c0
  [271641.879224] [c00000037540fd40] [c00000000014bba0] ___do_page_fault+0x1c0/0x980
  [271641.879228] [c00000037540fdf0] [c00000000014c44c] hash__do_page_fault+0x2c/0xc0
  [271641.879232] [c00000037540fe20] [c0000000001565d8] do_hash_fault+0x128/0x1d0
  [271641.879236] [c00000037540fe50] [c000000000008be0] data_access_common_virt+0x210/0x220
  [271641.879548] Tasks state (memory values in pages):
  ...
  [271641.879550] [  pid  ]   uid  tgid total_vm      rss rss_anon rss_file rss_shmem pgtables_bytes swapents oom_score_adj name
  [271641.879555] [ 177372]     0 177372      571        0        0        0         0    51200       96             0 test_zswap
  [271641.879562] oom-kill:constraint=CONSTRAINT_MEMCG,nodemask=(null),cpuset=/,mems_allowed=0,oom_memcg=/no_zswap_test,task_memcg=/no_zswap_test,task=test_zswap,pid=177372,uid=0
  [271641.879578] Memory cgroup out of memory: Killed process 177372 (test_zswap) total-vm:36544kB, anon-rss:0kB, file-rss:0kB, shmem-rss:0kB, UID:0 pgtables:50kB oom_score_adj:0

Link: https://lore.kernel.org/20260424040059.12940-3-li.wang@linux.dev
Signed-off-by: Li Wang <li.wang@linux.dev>
Acked-by: Yosry Ahmed <yosry@kernel.org>
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: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Jiayuan Chen <jiayuan.chen@linux.dev>
Cc: Waiman Long <longman@redhat.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions