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

[Michal Hocko <mhocko@kernel.org>]

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:
> 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.
> 
> Reported-by: Adric Blake <promarbler14@gmail.com>
> 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

-- 
Michal Hocko
SUSE Labs