linux-kernel - Re: WARNINGs in set_task_reclaim_state with memory cgroup and full memory usage

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Message-ID: <CALOAHbAtuQFB=GC41ZgSLXxheaEY4yz=fO9Zr5=rvTnyOYjF3A@mail.gmail.com>
Date:   Tue, 27 Aug 2019 19:56:16 +0800
From:   Yafang Shao <laoar.shao@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Yang Shi <yang.shi@...ux.alibaba.com>,
        Adric Blake <promarbler14@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: WARNINGs in set_task_reclaim_state with memory cgroup and full
 memory usage

On Tue, Aug 27, 2019 at 7:50 PM Michal Hocko <mhocko@...nel.org> wrote:
>
> On Tue 27-08-19 19:43:49, Yafang Shao wrote:
> > On Tue, Aug 27, 2019 at 6:43 PM Michal Hocko <mhocko@...nel.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.
>
> Why would it break it? Could you be more specific please?
>

Hmm, I have explained it when replying to  Hillf's patch.
The most suspcious one is settting target_mem_cgroup here, because we
only use it to judge whether it is in global reclaim.
While the memcg softlimit reclaim is really in global reclaims.

Another example the reclaim_idx, if is not same with reclaim_idx in
page allocation context, the reclaimed pages may not be used by the
allocator, especially in the direct reclaim.

And some other things in scan_control.

> > Anyway, we can send this patch as a standalong one.
> > Feel free to add:
> >
> > Acked-by: Yafang Shao <laoar.shao@...il.com>
>
> Thanks!
> --
> Michal Hocko
> SUSE Labs