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

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Date:   Fri, 30 Oct 2020 14:39:02 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Yang Shi <shy828301@...il.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>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] mm/compaction: count pages and stop correctly during page
 isolation.

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.

—
Best Regards,
Yan Zi

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