[PATCH v10] mm: vmscan: try to reclaim swapcache pages if no swap space

Liu Shixin
2023-11-21T08:08:16Z

When spaces of swap devices are exhausted, only file pages can be
reclaimed. But there are still some swapcache pages in anon lru list.
This can lead to a premature out-of-memory.

The problem is found with such step:

Firstly, set a 9MB disk swap space, then create a cgroup with 10MB
memory limit, then runs an program to allocates about 15MB memory.

The problem occurs occasionally, which may need about 100 times [1].

Fix it by checking number of swapcache pages in can_reclaim_anon_pages().
If the number is not zero, return true and set swapcache_only to 1.
When scan anon lru list in swapcache_only mode, non-swapcache pages will
be skipped to isolate in order to accelerate reclaim efficiency.

However, in swapcache_only mode, the scan count still increased when scan
non-swapcache pages because there are large number of non-swapcache pages
and rare swapcache pages in swapcache_only mode, and if the non-swapcache
is skipped and do not count, the scan of pages in isolate_lru_folios() can
eventually lead to hung task, just as Sachin reported [2].

By the way, since there are enough times of memory reclaim before OOM, it
is not need to isolate too much swapcache pages in one times.

[1]. https://lore.kernel.org/lkml/CAJD7tkZAfgncV+KbKr36=eDzMnT=9dZOT0dpMWcurHLr6Do+GA@mail.gmail.com/
[2]. https://lore.kernel.org/linux-mm/CAJD7tkafz_2XAuqE8tGLPEcpLngewhUo=5US14PAtSM9tLBUQg@mail.gmail.com/

Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Tested-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
---
v9->v10: Use per-node swapcache suggested by Yu Zhao.
v8->v9: Move the swapcache check after can_demote() and refector
can_reclaim_anon_pages() a bit.
v7->v8: Reset swapcache_only at the beginning of can_reclaim_anon_pages().
v6->v7: Reset swapcache_only to zero after there are swap spaces.
v5->v6: Fix NULL pointing derefence and hung task problem reported by Sachin.

mm/vmscan.c | 50 +++++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 49 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 506f8220c5fe..1fcc94717370 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -136,6 +136,9 @@ struct scan_control {
/* Always discard instead of demoting to lower tier memory */
unsigned int no_demotion:1;

+ /* Swap space is exhausted, only reclaim swapcache for anon LRU */
+ unsigned int swapcache_only:1;
+
/* Allocation order */
s8 order;

@@ -308,10 +311,36 @@ static bool can_demote(int nid, struct scan_control *sc)
return true;
}

+#ifdef CONFIG_SWAP
+static bool can_reclaim_swapcache(struct mem_cgroup *memcg, int nid)
+{
+ struct pglist_data *pgdat = NODE_DATA(nid);
+ unsigned long nr_swapcache;
+
+ if (!memcg) {
+ nr_swapcache = node_page_state(pgdat, NR_SWAPCACHE);
+ } else {
+ struct lruvec *lruvec = mem_cgroup_lruvec(memcg, pgdat);
+
+ nr_swapcache = lruvec_page_state_local(lruvec, NR_SWAPCACHE);
+ }
+
+ return nr_swapcache > 0;
+}
+#else
+static bool can_reclaim_swapcache(struct mem_cgroup *memcg, int nid)
+{
+ return false;
+}
+#endif
+
static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg,
int nid,
struct scan_control *sc)
{
+ if (sc)
+ sc->swapcache_only = 0;
+
if (memcg == NULL) {
/*
* For non-memcg reclaim, is there
@@ -330,7 +359,17 @@ static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg,
*
* Can it be reclaimed from this node via demotion?
*/
- return can_demote(nid, sc);
+ if (can_demote(nid, sc))
+ return true;
+
+ /* Is there any swapcache pages to reclaim in this node? */
+ if (can_reclaim_swapcache(memcg, nid)) {
+ if (sc)
+ sc->swapcache_only = 1;
+ return true;
+ }
+
+ return false;
}

/*
@@ -1642,6 +1681,15 @@ static unsigned long isolate_lru_folios(unsigned long nr_to_scan,
*/
scan += nr_pages;

+ /*
+ * Count non-swapcache too because the swapcache pages may
+ * be rare and it takes too much times here if not count
+ * the non-swapcache pages.
+ */
+ if (unlikely(sc->swapcache_only && !is_file_lru(lru) &&
+ !folio_test_swapcache(folio)))
+ goto move;
+
if (!folio_test_lru(folio))
goto move;
if (!sc->may_unmap && folio_mapped(folio))
--
2.25.1