[PATCH] zswap: do not shrink when memory.zswap.max is 0

[Nhat Pham]

Before storing a page, zswap first checks if the number of stored pages
exceeds the limit specified by memory.zswap.max, for each cgroup in the
hierarchy. If this limit is reached or exceeded, then zswap shrinking is
triggered and short-circuits the store attempt.

However, if memory.zswap.max = 0 for a cgroup, no amount of writeback
will allow future store attempts from processes in this cgroup to
succeed. Furthermore, this create a pathological behavior in a system
where some cgroups have memory.zswap.max = 0 and some do not: the
processes in the former cgroups, under memory pressure, will evict pages
stored by the latter continually, until the need for swap ceases or the
pool becomes empty.

As a result of this, we observe a disproportionate amount of zswap
writeback and a perpetually small zswap pool in our experiments, even
though the pool limit is never hit.

This patch fixes the issue by rejecting zswap store attempt without
shrinking the pool when memory.zswap.max is 0.

Fixes: f4840ccfca25 ("zswap: memcg accounting")
Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx>
---
 include/linux/memcontrol.h | 6 +++---
 mm/memcontrol.c            | 8 ++++----
 mm/zswap.c                 | 9 +++++++--
 3 files changed, 14 insertions(+), 9 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 222d7370134c..507bed3a28b0 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1899,13 +1899,13 @@ static inline void count_objcg_event(struct obj_cgroup *objcg,
 #endif /* CONFIG_MEMCG_KMEM */
 
 #if defined(CONFIG_MEMCG_KMEM) && defined(CONFIG_ZSWAP)
-bool obj_cgroup_may_zswap(struct obj_cgroup *objcg);
+int obj_cgroup_may_zswap(struct obj_cgroup *objcg);
 void obj_cgroup_charge_zswap(struct obj_cgroup *objcg, size_t size);
 void obj_cgroup_uncharge_zswap(struct obj_cgroup *objcg, size_t size);
 #else
-static inline bool obj_cgroup_may_zswap(struct obj_cgroup *objcg)
+static inline int obj_cgroup_may_zswap(struct obj_cgroup *objcg)
 {
-	return true;
+	return 0;
 }
 static inline void obj_cgroup_charge_zswap(struct obj_cgroup *objcg,
 					   size_t size)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 4b27e245a055..09aad0e6f2ea 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -7783,10 +7783,10 @@ static struct cftype memsw_files[] = {
  * spending cycles on compression when there is already no room left
  * or zswap is disabled altogether somewhere in the hierarchy.
  */
-bool obj_cgroup_may_zswap(struct obj_cgroup *objcg)
+int obj_cgroup_may_zswap(struct obj_cgroup *objcg)
 {
 	struct mem_cgroup *memcg, *original_memcg;
-	bool ret = true;
+	int ret = 0;
 
 	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
 		return true;
@@ -7800,7 +7800,7 @@ bool obj_cgroup_may_zswap(struct obj_cgroup *objcg)
 		if (max == PAGE_COUNTER_MAX)
 			continue;
 		if (max == 0) {
-			ret = false;
+			ret = -ENODEV;
 			break;
 		}
 
@@ -7808,7 +7808,7 @@ bool obj_cgroup_may_zswap(struct obj_cgroup *objcg)
 		pages = memcg_page_state(memcg, MEMCG_ZSWAP_B) / PAGE_SIZE;
 		if (pages < max)
 			continue;
-		ret = false;
+		ret = -ENOMEM;
 		break;
 	}
 	mem_cgroup_put(original_memcg);
diff --git a/mm/zswap.c b/mm/zswap.c
index 59da2a415fbb..7b13dc865438 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1175,8 +1175,13 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
 	}
 
 	objcg = get_obj_cgroup_from_page(page);
-	if (objcg && !obj_cgroup_may_zswap(objcg))
-		goto shrink;
+	if (objcg) {
+		ret = obj_cgroup_may_zswap(objcg);
+		if (ret == -ENODEV)
+			goto reject;
+		if (ret == -ENOMEM)
+			goto shrink;
+	}
 
 	/* reclaim space if needed */
 	if (zswap_is_full()) {
-- 
2.34.1