[PATCH v3 3/4] mm/zswap: Add per-memcg stat for proactive writeback
From: Hao Jia
Date: Tue May 26 2026 - 07:49:47 EST
From: Hao Jia <jiahao1@xxxxxxxxxxx>
Currently, zswap writeback can be triggered by either the pool limit
being hit or by the proactive writeback mechanism. However, the
existing 'zswpwb' metric in memory.stat and /proc/vmstat counts all
written back pages, making it difficult to distinguish between pages
written back due to the pool limit and those written back proactively.
Add a new statistic 'zswpwb_proactive' to memory.stat and /proc/vmstat.
This counter tracks the number of pages written back due to proactive
writeback. This allows users to better monitor and tune the proactive
writeback mechanism.
Signed-off-by: Hao Jia <jiahao1@xxxxxxxxxxx>
---
Documentation/admin-guide/cgroup-v2.rst | 4 +++
include/linux/vm_event_item.h | 1 +
mm/memcontrol.c | 1 +
mm/vmstat.c | 1 +
mm/zswap.c | 41 ++++++++++++++++++-------
5 files changed, 37 insertions(+), 11 deletions(-)
diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 6564abf0dec5..7d65aef83f7b 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1748,6 +1748,10 @@ The following nested keys are defined.
zswpwb
Number of pages written from zswap to swap.
+ zswpwb_proactive
+ Number of pages written from zswap to swap by proactive
+ writeback. This is a subset of zswpwb.
+
zswap_incomp
Number of incompressible pages currently stored in zswap
without compression. These pages could not be compressed to
diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
index 03fe95f5a020..7a5bee0a20b6 100644
--- a/include/linux/vm_event_item.h
+++ b/include/linux/vm_event_item.h
@@ -138,6 +138,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
ZSWPIN,
ZSWPOUT,
ZSWPWB,
+ ZSWPWB_PROACTIVE,
#endif
#ifdef CONFIG_X86
DIRECT_MAP_LEVEL2_SPLIT,
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e205e5de193d..7648b3fd940e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -571,6 +571,7 @@ static const unsigned int memcg_vm_event_stat[] = {
ZSWPIN,
ZSWPOUT,
ZSWPWB,
+ ZSWPWB_PROACTIVE,
#endif
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
THP_FAULT_ALLOC,
diff --git a/mm/vmstat.c b/mm/vmstat.c
index f534972f517d..66fd06d1bb01 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1452,6 +1452,7 @@ const char * const vmstat_text[] = {
[I(ZSWPIN)] = "zswpin",
[I(ZSWPOUT)] = "zswpout",
[I(ZSWPWB)] = "zswpwb",
+ [I(ZSWPWB_PROACTIVE)] = "zswpwb_proactive",
#endif
#ifdef CONFIG_X86
[I(DIRECT_MAP_LEVEL2_SPLIT)] = "direct_map_level2_splits",
diff --git a/mm/zswap.c b/mm/zswap.c
index 7bcbf788f634..b45d094f532a 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -160,6 +160,11 @@ struct zswap_pool {
char tfm_name[CRYPTO_MAX_ALG_NAME];
};
+struct zswap_shrink_walk_arg {
+ bool proactive;
+ bool encountered_page_in_swapcache;
+};
+
/* Global LRU lists shared by all zswap pools. */
static struct list_lru zswap_list_lru;
@@ -1042,7 +1047,8 @@ static bool zswap_decompress(struct zswap_entry *entry, struct folio *folio)
* freed.
*/
static int zswap_writeback_entry(struct zswap_entry *entry,
- swp_entry_t swpentry)
+ swp_entry_t swpentry,
+ bool proactive)
{
struct xarray *tree;
pgoff_t offset = swp_offset(swpentry);
@@ -1097,6 +1103,12 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
if (entry->objcg)
count_objcg_events(entry->objcg, ZSWPWB, 1);
+ if (proactive) {
+ count_vm_event(ZSWPWB_PROACTIVE);
+ if (entry->objcg)
+ count_objcg_events(entry->objcg, ZSWPWB_PROACTIVE, 1);
+ }
+
zswap_entry_free(entry);
/* folio is up to date */
@@ -1146,7 +1158,8 @@ static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_o
void *arg)
{
struct zswap_entry *entry = container_of(item, struct zswap_entry, lru);
- bool *encountered_page_in_swapcache = (bool *)arg;
+ struct zswap_shrink_walk_arg *walk_arg = arg;
+ bool proactive_wb = walk_arg && walk_arg->proactive;
swp_entry_t swpentry;
enum lru_status ret = LRU_REMOVED_RETRY;
int writeback_result;
@@ -1201,7 +1214,7 @@ static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_o
*/
spin_unlock(&l->lock);
- writeback_result = zswap_writeback_entry(entry, swpentry);
+ writeback_result = zswap_writeback_entry(entry, swpentry, proactive_wb);
if (writeback_result) {
zswap_reject_reclaim_fail++;
@@ -1212,9 +1225,9 @@ static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_o
* into the warmer region. We should terminate shrinking (if we're in the dynamic
* shrinker context).
*/
- if (writeback_result == -EEXIST && encountered_page_in_swapcache) {
+ if (writeback_result == -EEXIST && walk_arg) {
ret = LRU_STOP;
- *encountered_page_in_swapcache = true;
+ walk_arg->encountered_page_in_swapcache = true;
}
} else {
zswap_written_back_pages++;
@@ -1226,8 +1239,11 @@ static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_o
static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
struct shrink_control *sc)
{
+ struct zswap_shrink_walk_arg walk_arg = {
+ .proactive = false,
+ .encountered_page_in_swapcache = false,
+ };
unsigned long shrink_ret;
- bool encountered_page_in_swapcache = false;
if (!zswap_shrinker_enabled ||
!mem_cgroup_zswap_writeback_enabled(sc->memcg)) {
@@ -1236,9 +1252,9 @@ static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
}
shrink_ret = list_lru_shrink_walk(&zswap_list_lru, sc, &shrink_memcg_cb,
- &encountered_page_in_swapcache);
+ &walk_arg);
- if (encountered_page_in_swapcache)
+ if (walk_arg.encountered_page_in_swapcache)
return SHRINK_STOP;
return shrink_ret ? shrink_ret : SHRINK_STOP;
@@ -1709,7 +1725,10 @@ static long zswap_proactive_shrink_memcg(struct mem_cgroup *memcg,
return -ENOENT;
for_each_node_state(nid, N_NORMAL_MEMORY) {
- bool encountered_page_in_swapcache = false;
+ struct zswap_shrink_walk_arg walk_arg = {
+ .proactive = true,
+ .encountered_page_in_swapcache = false,
+ };
unsigned long nr_to_scan, nr_scanned = 0;
/*
@@ -1743,12 +1762,12 @@ static long zswap_proactive_shrink_memcg(struct mem_cgroup *memcg,
nr_written += list_lru_walk_one(&zswap_list_lru, nid, memcg,
&shrink_memcg_cb,
- &encountered_page_in_swapcache,
+ &walk_arg,
&nr_to_walk);
if (nr_written >= nr_to_write)
return nr_written;
- if (encountered_page_in_swapcache)
+ if (walk_arg.encountered_page_in_swapcache)
break;
cond_resched();
--
2.34.1