[PATCH v4 15/21] zswap: do not start zswap shrinker if there is no physical swap slots

[Nhat Pham <nphamcs@gmail.com>]

When swap is virtualized, we no longer pre-allocate a slot on swapfile
for each zswap entry. Do not start the zswap shrinker if there is no
physical swap slots available.

Signed-off-by: Nhat Pham <nphamcs@gmail.com>
---
 mm/zswap.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/zswap.c b/mm/zswap.c
index c5e1d252cb463..9d1822753d321 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1211,6 +1211,14 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
 	if (!zswap_shrinker_enabled || !mem_cgroup_zswap_writeback_enabled(memcg))
 		return 0;
 
+	/*
+	 * When swap is virtualized, we do not have any swap slots on swapfile
+	 * preallocated for zswap objects. If there is no slot available, we
+	 * cannot writeback and should just bail out here.
+	 */
+	if (!get_nr_swap_pages())
+		return 0;
+
 	/*
 	 * The shrinker resumes swap writeback, which will enter block
 	 * and may enter fs. XXX: Harmonize with vmscan.c __GFP_FS
-- 
2.52.0