[PATCH v2 08/12] mm/mglru: simplify and improve dirty writeback handling

Kairui Song via B4 Relay
2026-03-28T19:52:41Z

From: Kairui Song <kasong@tencent.com>

The current handling of dirty writeback folios is not working well for
file page heavy workloads: Dirty folios are protected and move to next
gen upon isolation of getting throttled or reactivation upon pageout
(shrink_folio_list).

This might help to reduce the LRU lock contention slightly, but as a
result, the ping-pong effect of folios between head and tail of last two
gens is serious as the shrinker will run into protected dirty writeback
folios more frequently compared to activation. The dirty flush wakeup
condition is also much more passive compared to active/inactive LRU.
Active / inactve LRU wakes the flusher if one batch of folios passed to
shrink_folio_list is unevictable due to under writeback, but MGLRU
instead has to check this after the whole reclaim loop is done, and then
count the isolation protection number compared to the total reclaim
number.

And we previously saw OOM problems with it, too, which were fixed but
still not perfect [1].

So instead, just drop the special handling for dirty writeback, just
re-activate it like active / inactive LRU. And also move the dirty flush
wake up check right after shrink_folio_list. This should improve both
throttling and performance.

Test with YCSB workloadb showed a major performance improvement:

Before this series:
Throughput(ops/sec): 61642.78008938203
AverageLatency(us): 507.11127774145166
pgpgin 158190589
pgpgout 5880616
workingset_refault 7262988

After this commit:
Throughput(ops/sec): 80216.04855744806 (+30.1%, higher is better)
AverageLatency(us): 388.17633477268913 (-23.5%, lower is better)
pgpgin 101871227 (-35.6%, lower is better)
pgpgout 5770028
workingset_refault 3418186 (-52.9%, lower is better)

The refault rate is ~50% lower, and throughput is ~30% higher, which
is a huge gain. We also observed significant performance gain for
other real-world workloads.

We were concerned that the dirty flush could cause more wear for SSD:
that should not be the problem here, since the wakeup condition is when
the dirty folios have been pushed to the tail of LRU, which indicates
that memory pressure is so high that writeback is blocking the workload
already.

Reviewed-by: Axel Rasmussen <axelrasmussen@google.com>
Link: https://lore.kernel.org/linux-mm/20241026115714.1437435-1-jingxiangzeng.cas@gmail.com/ [1]
Signed-off-by: Kairui Song <kasong@tencent.com>
---
mm/vmscan.c | 57 ++++++++++++++++-----------------------------------------
1 file changed, 16 insertions(+), 41 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 8de5c8d5849e..17b5318fad39 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4583,7 +4583,6 @@ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_c
int tier_idx)
{
bool success;
- bool dirty, writeback;
int gen = folio_lru_gen(folio);
int type = folio_is_file_lru(folio);
int zone = folio_zonenum(folio);
@@ -4633,21 +4632,6 @@ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_c
return true;
}

- dirty = folio_test_dirty(folio);
- writeback = folio_test_writeback(folio);
- if (type == LRU_GEN_FILE && dirty) {
- sc->nr.file_taken += delta;
- if (!writeback)
- sc->nr.unqueued_dirty += delta;
- }
-
- /* waiting for writeback */
- if (writeback || (type == LRU_GEN_FILE && dirty)) {
- gen = folio_inc_gen(lruvec, folio, true);
- list_move(&folio->lru, &lrugen->folios[gen][type][zone]);
- return true;
- }
-
return false;
}

@@ -4754,8 +4738,6 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, nr_to_scan,
scanned, skipped, isolated,
type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);
- if (type == LRU_GEN_FILE)
- sc->nr.file_taken += isolated;

*isolatedp = isolated;
return scanned;
@@ -4858,12 +4840,27 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
return scanned;
retry:
reclaimed = shrink_folio_list(&list, pgdat, sc, &stat, false, memcg);
- sc->nr.unqueued_dirty += stat.nr_unqueued_dirty;
sc->nr_reclaimed += reclaimed;
trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id,
type_scanned, reclaimed, &stat, sc->priority,
type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);

+ /*
+ * If too many file cache in the coldest generation can't be evicted
+ * due to being dirty, wake up the flusher.
+ */
+ if (stat.nr_unqueued_dirty == isolated) {
+ wakeup_flusher_threads(WB_REASON_VMSCAN);
+
+ /*
+ * For cgroupv1 dirty throttling is achieved by waking up
+ * the kernel flusher here and later waiting on folios
+ * which are in writeback to finish (see shrink_folio_list()).
+ */
+ if (!writeback_throttling_sane(sc))
+ reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK);
+ }
+
list_for_each_entry_safe_reverse(folio, next, &list, lru) {
DEFINE_MIN_SEQ(lruvec);

@@ -5020,28 +5017,6 @@ static bool try_to_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
cond_resched();
}

- /*
- * If too many file cache in the coldest generation can't be evicted
- * due to being dirty, wake up the flusher.
- */
- if (sc->nr.unqueued_dirty && sc->nr.unqueued_dirty == sc->nr.file_taken) {
- struct pglist_data *pgdat = lruvec_pgdat(lruvec);
-
- wakeup_flusher_threads(WB_REASON_VMSCAN);
-
- /*
- * For cgroupv1 dirty throttling is achieved by waking up
- * the kernel flusher here and later waiting on folios
- * which are in writeback to finish (see shrink_folio_list()).
- *
- * Flusher may not be able to issue writeback quickly
- * enough for cgroupv1 writeback throttling to work
- * on a large system.
- */
- if (!writeback_throttling_sane(sc))
- reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK);
- }
-
return need_rotate;
}

--
2.53.0