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/[email protected]
Signed-off-by: Li Wang <[email protected]>
Acked-by: Dan Streetman <[email protected]>
Cc: Seth Jennings <[email protected]>
Cc: Huang Ying <[email protected]>
Cc: Yu Zhao <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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 */