[PATCH v2 4/6] mm: zswap: make writeback run in the background

[Takero Funaki <flintglass@gmail.com>]

Drop the WQ_MEM_RECLAIM flag from the zswap global shrinker workqueue to
resolve resource contention with actual kernel memory reclaim.

The current zswap global shrinker and its writeback contend with acutual
memory reclaim, leading to system responsiveness issues when the zswap
writeback and direct reclaim run concurrently.  Unlike kernel memory
shrinkers, the global shrinker works in the background behind the zswap
pool, which acts as a large in-memory buffer. The zswap writeback is not
urgent and is not strictly necessary to reclaim kernel memory.
Even when zswap shrinker cannot evict pages, zswap_store() can reject
reclaimed pages, and the rejected pages have swap space preallocated.
Delaying writeback or shrinker progress do not interfere page reclaim.

The visible issue in the current implementation occurs when a large
amount of direct reclaim happens and zswap cannot store the incoming
pages. Both the zswap global shrinker and the memory reclaimer start
writing back pages concurrently. This leads the entire system
responsivility issue that does not occur without zswap.  The
shrink_worker() running on WQ_MEM_RECLAIM blocks other important works
required for memory reclamation. In this case, swp_writepage() and
zswap_writeback() are consuming time and contend with each other for
workqueue scheduling and I/O resources, especially on slow swap devices.

Note that this issue has been masked by the global shrinker failing to
evict a considerable number of pages. This patch is required to fix the
shrinker to continuously reduce the pool size to the acceptable
threshold.

The probability of this issue can be mitigated mostly by removing the
WQ_MEM_RECLAIM flag from the zswap shrinker workqueue. With this change,
the invocation of shrink_worker() and its writeback will be delayed
while reclamation is running on WQ_MEM_RECLAIM workqueue.

Signed-off-by: Takero Funaki <flintglass@gmail.com>
---
 mm/zswap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 24acbab44e7a..76691ca7b6a7 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1806,7 +1806,7 @@ static int zswap_setup(void)
 		goto hp_fail;
 
 	shrink_wq = alloc_workqueue("zswap-shrink",
-			WQ_UNBOUND|WQ_MEM_RECLAIM, 1);
+			WQ_UNBOUND, 1);
 	if (!shrink_wq)
 		goto shrink_wq_fail;
 
-- 
2.43.0