From: Kairui Song The forced protection of younger generations contradicts the goal of minimizing the I/O cost of reclaim. Under high memory pressure, MGLRU can repeatedly trigger premature OOMs, while the classic LRU does not. The problem is that aging may fail to keep up with reclaim. When this happens, reclaimers may all abort prematurely because they see only empty generations or just two generations remaining. Proactive aging does not solve this problem either. The number of threads performing proactive aging is limited, while the number of concurrent reclaimers can be much larger. This is a fundamental limitation of MGLRU. One possible solution is to use an aging lock. However, the aging lock introduces significant jitter under heavy memory pressure and has been reported to cause noticeable lags on Android. Furthermore, the generation-based reclaim model can conflict with the goal of swappiness, which is to minimize I/O cost rather than blindly follow folio hotness. The previous commit allowed reclaimers running at non-default priorities to reclaim up to `lrugen->max_seq`, while reclaimers at the default priority could reclaim only up to `lrugen->min_seq`. This patch further refines generation protection based on reclaim priority. At the default priority, reclaimers can reclaim generations down to `lrugen->min_seq - MIN_NR_GENS`. As reclaim priority increases, reclaimers are progressively allowed to reclaim younger generations. With this graduated protection, premature OOMs are no longer observed, while swappiness is also respected. Signed-off-by: Kairui Song Signed-off-by: Barry Song (Xiaomi) Reviewed-by: Kunwu Chan --- mm/vmscan.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index 88567002b7ef..fff7ee17eb0d 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -4767,12 +4767,14 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec, VM_WARN_ON_ONCE(!list_empty(list)); /* - * Respect the gen hotness distribution at low priority to avoid - * aging and scan cost, but prefer to satisfy the reclaimers request - * when under pressure to respect swappiness more and reduce IO cost. + * Respect hotness at default priority, and only reach into the + * newest generation when under severe pressure, so hot folios are + * not evicted as soon as priority is raised. */ if (sc->priority == DEF_PRIORITY) - seq = lrugen->min_seq[type]; + seq = lrugen->max_seq - MIN_NR_GENS; + else if (sc->priority > DEF_PRIORITY / 2) + seq = lrugen->max_seq - 1; else seq = lrugen->max_seq; -- 2.34.1