[PATCH 1/3] mm-unstable: Multi-gen LRU: Fix per-zone reclaim

From: Kalesh Singh
Date: Tue Aug 01 2023 - 20:20:06 EST

MGLRU has a LRU list for each zone for each type (anon/file) in each


The min_seq (oldest generation) can progress independently for each
type but the max_seq (youngest generation) is shared for both anon and
file. This is to maintain a common frame of reference.

In order for eviction to advance the min_seq of a type, all the per-zone
lists in the oldest generation of that type must be empty.

The eviction logic only considers pages from eligible zones for
eviction or promotion.

scan_folios() {
for (zone = sc->reclaim_idx; zone >= 0; zone--) {
sort_folio(); // Promote
isolate_folio(); // Evict

Consider the system has the movable zone configured and default 4
generations. The current state of the system is as shown below
(only illustrating one type for simplicity):

Type: ANON

Zone DMA32 Normal Movable Device

Gen 0 0 0 4GB 0

Gen 1 0 1GB 1MB 0

Gen 2 1MB 4GB 1MB 0

Gen 3 1MB 1MB 1MB 0

Now consider there is a GFP_KERNEL allocation request (eligible zone
index <= Normal), evict_folios() will return without doing any work
since there are no pages to scan in the eligible zones of the oldest
generation. Reclaim won't make progress until triggered from a ZONE_MOVABLE
allocation request; which may not happen soon if there is a lot of free
memory in the movable zone. This can lead to OOM kills, although there
is 1GB pages in the Normal zone of Gen 1 that we have not yet tried to

This issue is not seen in the conventional active/inactive LRU since
there are no per-zone lists.

If there are no (not enough) folios to scan in the eligible zones, move
folios from ineligible zone (zone_index > reclaim_index) to the next
generation. This allows for the progression of min_seq and reclaiming
from the next generation (Gen 1).

Qualcomm, Mediatek and raspberrypi [1] discovered this issue independently.

[1] https://github.com/raspberrypi/linux/issues/5395

Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Reported-by: Charan Teja Kalla <quic_charante@xxxxxxxxxxx>
Reported-by: Lecopzer Chen <lecopzer.chen@xxxxxxxxxxxx>
Signed-off-by: Kalesh Singh <kaleshsingh@xxxxxxxxxx>
mm/vmscan.c | 18 ++++++++++++++----
1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 4039620d30fe..489a4fc7d9b1 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4889,7 +4889,8 @@ static int lru_gen_memcg_seg(struct lruvec *lruvec)
* the eviction

-static bool sort_folio(struct lruvec *lruvec, struct folio *folio, int tier_idx)
+static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_control *sc,
+ int tier_idx)
bool success;
int gen = folio_lru_gen(folio);
@@ -4939,6 +4940,13 @@ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, int tier_idx)
return true;

+ /* ineligible */
+ if (zone > sc->reclaim_idx) {
+ gen = folio_inc_gen(lruvec, folio, false);
+ list_move_tail(&folio->lru, &lrugen->folios[gen][type][zone]);
+ return true;
+ }
/* waiting for writeback */
if (folio_test_locked(folio) || folio_test_writeback(folio) ||
(type == LRU_GEN_FILE && folio_test_dirty(folio))) {
@@ -4987,7 +4995,8 @@ static bool isolate_folio(struct lruvec *lruvec, struct folio *folio, struct sca
static int scan_folios(struct lruvec *lruvec, struct scan_control *sc,
int type, int tier, struct list_head *list)
- int gen, zone;
+ int i;
+ int gen;
enum vm_event_item item;
int sorted = 0;
int scanned = 0;
@@ -5003,9 +5012,10 @@ static int scan_folios(struct lruvec *lruvec, struct scan_control *sc,

gen = lru_gen_from_seq(lrugen->min_seq[type]);

- for (zone = sc->reclaim_idx; zone >= 0; zone--) {
+ for (i = MAX_NR_ZONES; i > 0; i--) {
int skipped = 0;
+ int zone = (sc->reclaim_idx + i) % MAX_NR_ZONES;
struct list_head *head = &lrugen->folios[gen][type][zone];

while (!list_empty(head)) {
@@ -5019,7 +5029,7 @@ static int scan_folios(struct lruvec *lruvec, struct scan_control *sc,

scanned += delta;

- if (sort_folio(lruvec, folio, tier))
+ if (sort_folio(lruvec, folio, sc, tier))
sorted += delta;
else if (isolate_folio(lruvec, folio, sc)) {
list_add(&folio->lru, list);