linux-kernel - Re: [PATCH] mm/compaction: count pages and stop correctly during page isolation.

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Message-ID: <e0f38829-8e47-62d7-924a-920096aae739@suse.cz>
Date:   Fri, 30 Oct 2020 15:55:45 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...e.com>, Zi Yan <ziy@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Rik van Riel <riel@...riel.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/compaction: count pages and stop correctly during page
 isolation.

On 10/30/20 3:49 PM, Michal Hocko wrote:
> On Fri 30-10-20 10:35:43, Zi Yan wrote:
>> On 30 Oct 2020, at 9:36, Michal Hocko wrote:
>> 
>> > On Fri 30-10-20 08:20:50, Zi Yan wrote:
>> >> On 30 Oct 2020, at 5:43, Michal Hocko wrote:
>> >>
>> >>> [Cc Vlastimil]
>> >>>
>> >>> On Thu 29-10-20 16:04:35, Zi Yan wrote:
>> >>>
>> >>> Does thp_nr_pages work for __PageMovable pages?
>> >>
>> >> Yes. It is the same as compound_nr() but compiled
>> >> to 1 when THP is not enabled.
>> >
>> > I am sorry but I do not follow. First of all the implementation of the
>> > two is different and also I was asking about __PageMovable which should
>> > never be THP IIRC. Can they be compound though?
>> 
>> __PageMovable, non-lru movable pages, can be compound and thp_nr_page cannot
>> be used for it, since when THP is off, thp_nr_page will return the wrong number.
>> I got confused by its name, sorry.
> 
> OK, this matches my understanding. Good we are on the same page.
> 
>> But __PageMovable is irrelevant to this patch, since we are using
>> __isolate_lru_page to isolate pages. non-lru __PageMovable should not appear
>> after isolate_succes. thp_nr_pages can be used here.
> 
> But this is still not clear to me. __PageMovable pages are isolated by
> isolate_movable_page and then jump to isolate_succes. Does that somehow
> changes the nature of the page being compound or tat thp_nr_page would
> start working on those pages.

Agreed that page movable can appear after isolate_success. compound_nr() should 
work for both.
Note that too_many_isolated() doesn't see __PageMovable isolated pages, as they 
are not counted as NR_ISOLATED_FILE/NR_ISOLATED_ANON, AFAIK. So in that sense 
they are irrelevant to the bug at hand... for now.