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

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Message-ID: <6CAAB1FC-2B41-490B-A67A-93063629C19B@nvidia.com>
Date:   Fri, 30 Oct 2020 08:20:50 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Michal Hocko <mhocko@...e.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        Rik van Riel <riel@...riel.com>,
        <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 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.

—
Best Regards,
Yan Zi

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