Re: WARNINGs in set_task_reclaim_state with memory cgroup and full memory usage

On 8/27/19 3:43 AM, Michal Hocko wrote:

If there are no objection to the patch I will post it as a standalong
one.

On Mon 26-08-19 12:55:21, Michal Hocko wrote:

quoted

 From 59d128214a62bf2d83c2a2a9cde887b4817275e7 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@suse.com>
Date: Mon, 26 Aug 2019 12:43:15 +0200
Subject: [PATCH] mm, memcg: do not set reclaim_state on soft limit reclaim

Adric Blake has noticed the following warning:
[38491.963105] WARNING: CPU: 7 PID: 175 at mm/vmscan.c:245 set_task_reclaim_state+0x1e/0x40
[...]
[38491.963239] Call Trace:
[38491.963246]  mem_cgroup_shrink_node+0x9b/0x1d0
[38491.963250]  mem_cgroup_soft_limit_reclaim+0x10c/0x3a0
[38491.963254]  balance_pgdat+0x276/0x540
[38491.963258]  kswapd+0x200/0x3f0
[38491.963261]  ? wait_woken+0x80/0x80
[38491.963265]  kthread+0xfd/0x130
[38491.963267]  ? balance_pgdat+0x540/0x540
[38491.963269]  ? kthread_park+0x80/0x80
[38491.963273]  ret_from_fork+0x35/0x40
[38491.963276] ---[ end trace 727343df67b2398a ]---

which tells us that soft limit reclaim is about to overwrite the
reclaim_state configured up in the call chain (kswapd in this case but
the direct reclaim is equally possible). This means that reclaim stats
would get misleading once the soft reclaim returns and another reclaim
is done.

Fix the warning by dropping set_task_reclaim_state from the soft reclaim
which is always called with reclaim_state set up.

This is exactly what I thought. Looks good to me. Acked-by: Yang Shi 
[off-list ref]

quoted

Reported-by: Adric Blake <redacted>
Signed-off-by: Michal Hocko <mhocko@suse.com>
---
  mm/vmscan.c | 5 +++--
  1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index c77d1e3761a7..a6c5d0b28321 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c

@@ -3220,6 +3220,7 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
  
  #ifdef CONFIG_MEMCG
  
+/* Only used by soft limit reclaim. Do not reuse for anything else. */
  unsigned long mem_cgroup_shrink_node(struct mem_cgroup *memcg,
  						gfp_t gfp_mask, bool noswap,
  						pg_data_t *pgdat,

@@ -3235,7 +3236,8 @@ unsigned long mem_cgroup_shrink_node(struct mem_cgroup *memcg,
  	};
  	unsigned long lru_pages;
  
-	set_task_reclaim_state(current, &sc.reclaim_state);
+	WARN_ON_ONCE(!current->reclaim_state);
+
  	sc.gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
  			(GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK);
  

@@ -3253,7 +3255,6 @@ unsigned long mem_cgroup_shrink_node(struct mem_cgroup *memcg,
  
  	trace_mm_vmscan_memcg_softlimit_reclaim_end(sc.nr_reclaimed);
  
-	set_task_reclaim_state(current, NULL);
  	*nr_scanned = sc.nr_scanned;
  
  	return sc.nr_reclaimed;

-- 
2.20.1

-- 
Michal Hocko
SUSE Labs