During page/folio reclaim, we check folio is referenced using
folio_referenced() to avoid reclaiming folios that have been recently
accessed (hot memory). The ratinale is that this memory is likely to be
accessed soon, and hence reclaiming it will cause a refault.
For memcg reclaim, we pass in sc->target_mem_cgroup to
folio_referenced(), which means we only check accesses to the folio
from processes in the subtree of the target memcg. This behavior was
originally introduced by commit bed7161a519a ("Memory controller: make
page_referenced() cgroup aware") a long time ago. Back then, refaulted
pages would get charged to the memcg of the process that was faulting them
in. It made sense to only consider accesses coming from processes in the
subtree of target_mem_cgroup. If a page was charged to memcg A but only
being accessed by a sibling memcg B, we would reclaim it if memcg A is
under pressure. memcg B can then fault it back in and get charged for it
appropriately.
Today, this behavior still makes sense for file pages. However, unlike
file pages, when swapbacked pages are refaulted they are charged to the
memcg that was originally charged for them during swapout. Which
means that if a swapbacked page is charged to memcg A but only used by
memcg B, and we reclaim it when memcg A is under pressure, it would
simply be faulted back in and charged again to memcg A once memcg B
accesses it. In that sense, accesses from all memcgs matter equally when
considering if a swapbacked page/folio is a viable reclaim target.
Add folio_referenced_memcg() which decides what memcg we should pass to
folio_referenced() based on the folio type, and includes an elaborate
comment about why we should do so. This should help reclaim make better
decision and reduce refaults when reclaiming swapbacked memory that is
used by multiple memcgs.
Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
---
mm/vmscan.c | 38 ++++++++++++++++++++++++++++++++++----
1 file changed, 34 insertions(+), 4 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index c5a4bff11da6..f9fa0f9287e5 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1443,14 +1443,43 @@ enum folio_references {
FOLIOREF_ACTIVATE,
};
+/* What memcg should we pass to folio_referenced()? */
+static struct mem_cgroup *folio_referenced_memcg(struct folio *folio,
+ struct mem_cgroup *target_memcg)
+{
+ /*
+ * We check references to folios to make sure we don't reclaim hot
+ * folios that are likely to be refaulted soon. We pass a memcg to
+ * folio_referenced() to only check references coming from processes in
+ * that memcg's subtree.
+ *
+ * For file folios, we only consider references from processes in the
+ * subtree of the target memcg. If a folio is charged to
+ * memcg A but is only referenced by processes in memcg B, we reclaim it
+ * if memcg A is under pressure. If it is later accessed by memcg B it
+ * will be faulted back in and charged to memcg B. For memcg A, this is
+ * called memory that should be reclaimed.
+ *
+ * On the other hand, when swapbacked folios are faulted in, they get
+ * charged to the memcg that was originally charged for them at the time
+ * of swapping out. This means that if a folio that is charged to
+ * memcg A gets swapped out, it will get charged back to A when *any*
+ * memcg accesses it. In that sense, we need to consider references from
+ * *all* processes when considering whether to reclaim a swapbacked
+ * folio.
+ */
+ return folio_test_swapbacked(folio) ? NULL : target_memcg;
+}
+
static enum folio_references folio_check_references(struct folio *folio,
struct scan_control *sc)
{
int referenced_ptes, referenced_folio;
unsigned long vm_flags;
+ struct mem_cgroup *memcg = folio_referenced_memcg(folio,
+ sc->target_mem_cgroup);
- referenced_ptes = folio_referenced(folio, 1, sc->target_mem_cgroup,
- &vm_flags);
+ referenced_ptes = folio_referenced(folio, 1, memcg, &vm_flags);
referenced_folio = folio_test_clear_referenced(folio);
/*
@@ -2581,6 +2610,7 @@ static void shrink_active_list(unsigned long nr_to_scan,
while (!list_empty(&l_hold)) {
struct folio *folio;
+ struct mem_cgroup *memcg;
cond_resched();
folio = lru_to_folio(&l_hold);
@@ -2600,8 +2630,8 @@ static void shrink_active_list(unsigned long nr_to_scan,
}
/* Referenced or rmap lock contention: rotate */
- if (folio_referenced(folio, 0, sc->target_mem_cgroup,
- &vm_flags) != 0) {
+ memcg = folio_referenced_memcg(folio, sc->target_mem_cgroup);
+ if (folio_referenced(folio, 0, memcg, &vm_flags) != 0) {
/*
* Identify referenced, file-backed active folios and
* give them one more trip around the active list. So
--
2.38.0.rc1.362.ged0d419d3c-goog
