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Subject[PATCH v2] zswap: re-check zswap_is_full after do zswap_shrink
Date
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 in
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 setting to 0 via kernel parameter, the
zswap_is_full() will always return true to do zswap_shrink(). But if
the shinking is able to reclain a page successful, then proceeds to
compress/store another page, so the value of stored_pages will keep
changing.

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

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 | 9 +++++++++
1 file changed, 9 insertions(+)

diff --git a/mm/zswap.c b/mm/zswap.c
index 61a5c41..fd320c3 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 */
--
2.9.5
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