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

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Date:   Mon, 2 Nov 2020 14:03:25 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Yang Shi <shy828301@...il.com>, Zi Yan <ziy@...dia.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>, Rik van Riel <riel@...riel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH] mm/compaction: count pages and stop correctly during page
 isolation.

On 10/30/20 7:55 PM, Yang Shi wrote:
> On Fri, Oct 30, 2020 at 11:39 AM Zi Yan <ziy@...dia.com> wrote:
>>
>> On 30 Oct 2020, at 14:33, Yang Shi wrote:
>>
>> > On Fri, Oct 30, 2020 at 6:36 AM Michal Hocko <mhocko@...e.com> 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:
>> >>>>> From: Zi Yan <ziy@...dia.com>
>> >>>>>
>> >>>>> In isolate_migratepages_block, when cc->alloc_contig is true, we are
>> >>>>> able to isolate compound pages, nr_migratepages and nr_isolated did not
>> >>>>> count compound pages correctly, causing us to isolate more pages than we
>> >>>>> thought. Use thp_nr_pages to count pages. Otherwise, we might be trapped
>> >>>>> in too_many_isolated while loop, since the actual isolated pages can go
>> >>>>> up to COMPACT_CLUSTER_MAX*512=16384, where COMPACT_CLUSTER_MAX is 32,
>> >>>>> since we stop isolation after cc->nr_migratepages reaches to
>> >>>>> COMPACT_CLUSTER_MAX.
>> >>>>>
>> >>>>> In addition, after we fix the issue above, cc->nr_migratepages could
>> >>>>> never be equal to COMPACT_CLUSTER_MAX if compound pages are isolated,
>> >>>>> thus page isolation could not stop as we intended. Change the isolation
>> >>>>> stop condition to >=.
>> >>>>>
>> >>>>> Signed-off-by: Zi Yan <ziy@...dia.com>
>> >>>>> ---
>> >>>>>  mm/compaction.c | 8 ++++----
>> >>>>>  1 file changed, 4 insertions(+), 4 deletions(-)
>> >>>>>
>> >>>>> diff --git a/mm/compaction.c b/mm/compaction.c
>> >>>>> index ee1f8439369e..0683a4999581 100644
>> >>>>> --- a/mm/compaction.c
>> >>>>> +++ b/mm/compaction.c
>> >>>>> @@ -1012,8 +1012,8 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>> >>>>>
>> >>>>>  isolate_success:
>> >>>>>            list_add(&page->lru, &cc->migratepages);
>> >>>>> -          cc->nr_migratepages++;
>> >>>>> -          nr_isolated++;
>> >>>>> +          cc->nr_migratepages += thp_nr_pages(page);
>> >>>>> +          nr_isolated += thp_nr_pages(page);
>> >>>>
>> >>>> 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?
>> >
>> > I have the same question, can they be compound? If they can be
>> > compound, PageTransHuge() can't tell from THP and compound movable
>> > page, right?
>>
>> Right. I have updated the patch and use compound_nr instead.
> 
> Thanks. Actually I'm wondering what kind of movable page could be
> compound. Any real examples?

Looks like there's currently none. Compaction also wouldn't work properly with 
movable pages with order>0 as the free page scanner looks for order-0 pages 
only. But it won't hurt to use compound_nr() anyway.

>>
>> —
>> Best Regards,
>> Yan Zi
>