From 6b426d071419a40f61fe41fe1bd9e1b4fa5aeb37 Mon Sep 17 00:00:00 2001 From: Mina Almasry Date: Thu, 1 Dec 2022 15:33:17 -0800 Subject: [PATCH] mm: disable top-tier fallback to reclaim on proactive reclaim Reclaiming directly from top tier nodes breaks the aging pipeline of memory tiers. If we have a RAM -> CXL -> storage hierarchy, we should demote from RAM to CXL and from CXL to storage. If we reclaim a page from RAM, it means we 'demote' it directly from RAM to storage, bypassing potentially a huge amount of pages colder than it in CXL. However disabling reclaim from top tier nodes entirely would cause ooms in edge scenarios where lower tier memory is unreclaimable for whatever reason, e.g. memory being mlocked() or too hot to reclaim. In these cases we would rather the job run with a performance regression rather than it oom altogether. However, we can disable reclaim from top tier nodes for proactive reclaim. That reclaim is not real memory pressure, and we don't have any cause to be breaking the aging pipeline. [akpm@linux-foundation.org: restore comment layout, per Ying Huang] Link: https://lkml.kernel.org/r/20221201233317.1394958-1-almasrymina@google.com Signed-off-by: Mina Almasry Reviewed-by: "Huang, Ying" Reviewed-by: Yang Shi Cc: Greg Thelen Cc: Shakeel Butt Cc: Tim Chen Cc: Wei Xu Cc: Yosry Ahmed Signed-off-by: Andrew Morton --- mm/vmscan.c | 25 ++++++++++++++++++++++--- 1 file changed, 22 insertions(+), 3 deletions(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index dcd476a66a59..1a59171c6695 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -2088,10 +2088,29 @@ keep: nr_reclaimed += demote_folio_list(&demote_folios, pgdat); /* Folios that could not be demoted are still in @demote_folios */ if (!list_empty(&demote_folios)) { - /* Folios which weren't demoted go back on @folio_list for retry: */ + /* Folios which weren't demoted go back on @folio_list */ list_splice_init(&demote_folios, folio_list); - do_demote_pass = false; - goto retry; + + /* + * goto retry to reclaim the undemoted folios in folio_list if + * desired. + * + * Reclaiming directly from top tier nodes is not often desired + * due to it breaking the LRU ordering: in general memory + * should be reclaimed from lower tier nodes and demoted from + * top tier nodes. + * + * However, disabling reclaim from top tier nodes entirely + * would cause ooms in edge scenarios where lower tier memory + * is unreclaimable for whatever reason, eg memory being + * mlocked or too hot to reclaim. We can disable reclaim + * from top tier nodes in proactive reclaim though as that is + * not real memory pressure. + */ + if (!sc->proactive) { + do_demote_pass = false; + goto retry; + } } pgactivate = stat->nr_activate[0] + stat->nr_activate[1];