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

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Date:   Fri, 30 Oct 2020 14:36:25 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     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, Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] mm/compaction: count pages and stop correctly during
 page isolation.

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?

-- 
Michal Hocko
SUSE Labs