linux-kernel - Re: [RFC PATCH v2 5/5] mm: Split move_pages_to

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Date:   Thu, 20 Aug 2020 10:15:17 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Alex Shi <alex.shi@...ux.alibaba.com>
Cc:     Yang Shi <yang.shi@...ux.alibaba.com>,
        kbuild test robot <lkp@...el.com>,
        Rong Chen <rong.a.chen@...el.com>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        linux-mm <linux-mm@...ck.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Tejun Heo <tj@...nel.org>, cgroups@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Wei Yang <richard.weiyang@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [RFC PATCH v2 5/5] mm: Split move_pages_to_lru into 3 separate passes

On Thu, Aug 20, 2020 at 2:58 AM Alex Shi <alex.shi@...ux.alibaba.com> wrote:
>
>
>
> 在 2020/8/19 下午10:42, Alexander Duyck 写道:
> >> It's actually changed the meaning from current func. which I had seen a bug if no relock.
> >> but after move to 5.9 kernel, I can not reprodce the bug any more. I am not sure if 5.9 fixed
> >> the problem, and we don't need relock here.
> > So I am not sure what you mean here about "changed the meaning from
> > the current func". Which function are you referring to and what
> > changed?
> >
> > From what I can tell the pages cannot change memcg because they were
> > isolated and had the LRU flag stripped. They shouldn't be able to
> > change destination LRU vector as a result. Assuming that, then they
> > can all be processed under same LRU lock and we can avoid having to
> > release it until we are forced to do so to call putback_lru_page or
> > destroy the compound pages that were freed while we were shrinking the
> > LRU lists.
> >
>
> I had sent a bug which base on 5.8 kernel.
> https://lkml.org/lkml/2020/7/28/465
>
> I am not sure it was fixed in new kernel. The original line was introduced by Hugh Dickins
> I believe it would be great if you can get comments from him.

When I brought this up before you had pointed to the relocking being
due to the fact that the function was reacquiring the lruvec for some
reason. I wonder if the fact that the LRU bit stripping serializing
things made it so that the check for the lruvec after releasing the
lock became redundant.

- Alex