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

[Yafang Shao <laoar.shao@gmail.com>]

On Tue, Aug 27, 2019 at 6:43 PM Michal Hocko <mhocko@kernel.org> wrote:
>
> If there are no objection to the patch I will post it as a standalong
> one.

I have no objection to your patch. It could fix the issue.

I still think that it is not proper to use a new scan_control here as
it breaks the global reclaim context.
This context switch from global reclaim to memcg reclaim is very
subtle change to the subsequent processing, that may cause some
unexpected behavior.
Anyway, we can send this patch as a standalong one.
Feel free to add:

Acked-by: Yafang Shao <laoar.shao@gmail.com>

>
> 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