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

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Message-ID: <400B3460-65C0-4C48-A7EA-1A9F5780EC9C@nvidia.com>
Date:   Fri, 30 Oct 2020 10:35:43 -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 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:
>>>> 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?

__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.

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.

—
Best Regards,
Yan Zi

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