zswap: re-check zswap_is_full() after do zswap_shrink()

/sys/../zswap/stored_pages keeps rising in a zswap test with
"zswap.max_pool_percent=0" parameter.  But it should not compress or
store pages any more since there is no space in the compressed pool.

Reproduce steps:
  1. Boot kernel with "zswap.enabled=1"
  2. Set the max_pool_percent to 0
      # echo 0 > /sys/module/zswap/parameters/max_pool_percent
  3. Do memory stress test to see if some pages have been compressed
      # stress --vm 1 --vm-bytes $mem_available"M" --timeout 60s
  4. Watching the 'stored_pages' number increasing or not

The root cause is:

  When zswap_max_pool_percent is set to 0 via kernel parameter,
  zswap_is_full() will always return true due to zswap_shrink().  But if
  the shinking is able to reclain a page successfully the code then
  proceeds to compressing/storing another page, so the value of
  stored_pages will keep changing.

To solve the issue, this patch adds a zswap_is_full() check again after
  zswap_shrink() to make sure it's now under the max_pool_percent, and to
  not compress/store if we reached the limit.

Link: http://lkml.kernel.org/r/20180530103936.17812-1-liwang@redhat.com
Signed-off-by: Li Wang <liwang@redhat.com>
Acked-by: Dan Streetman <ddstreet@ieee.org>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Huang Ying <huang.ying.caritas@gmail.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 0 deletions

diff --git a/mm/zswap.c b/mm/zswap.c
index 7d34e69507e3..cd91fd9d96b8 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1026,6 +1026,15 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
                         ret = -ENOMEM;
                         goto reject;
                 }
+
+                /* A second zswap_is_full() check after
+                 * zswap_shrink() to make sure it's now
+                 * under the max_pool_percent
+                 */
+                if (zswap_is_full()) {
+                        ret = -ENOMEM;
+                        goto reject;
+                }
         }
 
         /* allocate entry */