[PATCH RFC] zswap: reject to compress/store page if zswap_max_pool_percent is 0

[Li Wang <liwang@redhat.com>]

The '/sys/../zswap/stored_pages:' keep raising in zswap test with
"zswap.max_pool_percent=0" parameter. But theoretically, it should
not compress or store pages any more since there is no space for
compressed pool.

Reproduce steps:

  1. Boot kernel with "zswap.enabled=1 zswap.max_pool_percent=17"
  2. Set the max_pool_percent to 0
      # echo 0 > /sys/module/zswap/parameters/max_pool_percent
     Confirm this parameter works fine
      # cat /sys/kernel/debug/zswap/pool_total_size
      0
  3. Do memory stress test to see if some pages have been compressed
      # stress --vm 1 --vm-bytes $mem_available"M" --timeout 60s
     Watching the 'stored_pages' numbers increasing or not

The root cause is:

  When the zswap_max_pool_percent is set to 0 via kernel parameter, the zswap_is_full()
  will always return true to shrink the pool size by zswap_shrink(). If the pool size
  has been shrinked a little success, zswap will do compress/store pages again. Then we
  get fails on that as above.

Signed-off-by: Li Wang <liwang@redhat.com>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Huang Ying <huang.ying.caritas@gmail.com>
Cc: Yu Zhao <yuzhao@google.com>
---
 mm/zswap.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/zswap.c b/mm/zswap.c
index 61a5c41..2b537bb 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1007,6 +1007,11 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
 	u8 *src, *dst;
 	struct zswap_header zhdr = { .swpentry = swp_entry(type, offset) };
 
+	if (!zswap_max_pool_percent) {
+		ret = -ENOMEM;
+		goto reject;
+	}
+
 	/* THP isn't supported */
 	if (PageTransHuge(page)) {
 		ret = -EINVAL;
-- 
2.9.5