Il 02/08/23 04:56, Kalesh Singh ha scritto:

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

long nr_pages[MAX_NR_GENS][ANON_AND_FILE][MAX_NR_ZONES]; 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 reclaim.

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

Fixes: ac35a4902374 ("mm: multi-gen LRU: minimal implementation") Cc: stable@vger.kernel.org Cc: Yu Zhao yuzhao@google.com Cc: Andrew Morton akpm@linux-foundation.org Reported-by: Charan Teja Kalla quic_charante@quicinc.com Reported-by: Lecopzer Chen lecopzer.chen@mediatek.com Signed-off-by: Kalesh Singh kaleshsingh@google.com

Whole series tested on MT8173 Elm Chromebook and MT6795 Xperia M5 as those are low ram devices. Can't reproduce the issue described in your [1] link from RPi.

MediaTek: Tested-by: AngeloGioacchino Del Regno angelogioacchino.delregno@collabora.com

---

Changes in v2:

• Add Fixes tag and cc stable

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--) { LIST_HEAD(moved); 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);