From: =?utf-8?b?TWljaGFsIEtvdXRuw70gPG1rb3V0bnlAc3VzZS5jb20+?=
Date: Fri, 31 Jul 2020 14:22:51 +0200
Subject: mm: Fix protection usage propagation
Patch-mainline: v5.9-rc1
Git-commit: a6f23d14ec7d7d02220ad8bb2774be3322b9aeec
References: bsc#1174002

When workload runs in cgroups that aren't directly below root cgroup and
their parent specifies reclaim protection, it may end up ineffective.

The reason is that propagate_protected_usage() is not called in all
hierarchy up. All the protected usage is incorrectly accumulated in the
workload's parent. This means that siblings_low_usage is overestimated
and effective protection underestimated. Even though it is transitional
phenomenon (uncharge path does correct propagation and fixes the wrong
children_low_usage), it can undermine the indended protection
unexpectedly.

The fix is simply updating children_low_usage in respective ancestors
also in the charging path.

Fixes: 230671533d64 ("mm: memory.low hierarchical behavior")
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Michal Hocko <mhocko@suse.com>
---
 mm/page_counter.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/page_counter.c b/mm/page_counter.c
index de31470655f6..147ff99187b8 100644
--- a/mm/page_counter.c
+++ b/mm/page_counter.c
@@ -77,7 +77,7 @@ void page_counter_charge(struct page_counter *counter, unsigned long nr_pages)
 		long new;
 
 		new = atomic_long_add_return(nr_pages, &c->usage);
-		propagate_protected_usage(counter, new);
+		propagate_protected_usage(c, new);
 		/*
 		 * This is indeed racy, but we can live with some
 		 * inaccuracy in the watermark.
@@ -121,7 +121,7 @@ bool page_counter_try_charge(struct page_counter *counter,
 		new = atomic_long_add_return(nr_pages, &c->usage);
 		if (new > c->max) {
 			atomic_long_sub(nr_pages, &c->usage);
-			propagate_protected_usage(counter, new);
+			propagate_protected_usage(c, new);
 			/*
 			 * This is racy, but we can live with some
 			 * inaccuracy in the failcnt.
@@ -130,7 +130,7 @@ bool page_counter_try_charge(struct page_counter *counter,
 			*fail = c;
 			goto failed;
 		}
-		propagate_protected_usage(counter, new);
+		propagate_protected_usage(c, new);
 		/*
 		 * Just like with failcnt, we can live with some
 		 * inaccuracy in the watermark.