Re: Possible mem cgroup bug in kernels between 4.18.0 and 5.3-rc1.

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

Masoud, will you try this patch?

By the way, is /sys/fs/cgroup/memory/leaker/memory.usage_in_bytes remains non-zero
despite /sys/fs/cgroup/memory/leaker/tasks became empty due to memcg OOM killer expected?
Deleting big-data-file.bin after memcg OOM killer reduces some, but still remains
non-zero.

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From 2f92c70f390f42185c6e2abb8dda98b1b7d02fa9 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Date: Sun, 4 Aug 2019 00:41:30 +0900
Subject: [PATCH] memcg, oom: don't require __GFP_FS when invoking memcg OOM killer

Masoud Sharbiani noticed that commit 29ef680ae7c21110 ("memcg, oom: move
out_of_memory back to the charge path") broke memcg OOM called from
__xfs_filemap_fault() path. It turned out that try_chage() is retrying
forever without making forward progress because mem_cgroup_oom(GFP_NOFS)
cannot invoke the OOM killer due to commit 3da88fb3bacfaa33 ("mm, oom:
move GFP_NOFS check to out_of_memory"). Regarding memcg OOM, we need to
bypass GFP_NOFS check in order to guarantee forward progress.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: Masoud Sharbiani <msharbiani@apple.com>
Bisected-by: Masoud Sharbiani <msharbiani@apple.com>
Fixes: 29ef680ae7c21110 ("memcg, oom: move out_of_memory back to the charge path")
---
 mm/oom_kill.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index eda2e2a..26804ab 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -1068,9 +1068,10 @@ bool out_of_memory(struct oom_control *oc)
 	 * The OOM killer does not compensate for IO-less reclaim.
 	 * pagefault_out_of_memory lost its gfp context so we have to
 	 * make sure exclude 0 mask - all other users should have at least
-	 * ___GFP_DIRECT_RECLAIM to get here.
+	 * ___GFP_DIRECT_RECLAIM to get here. But mem_cgroup_oom() has to
+	 * invoke the OOM killer even if it is a GFP_NOFS allocation.
 	 */
-	if (oc->gfp_mask && !(oc->gfp_mask & __GFP_FS))
+	if (oc->gfp_mask && !(oc->gfp_mask & __GFP_FS) && !is_memcg_oom(oc))
 		return true;
 
 	/*
-- 
1.8.3.1