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

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Date:   Fri, 30 Oct 2020 15:49:17 +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 10:35:43, Zi Yan wrote:
> 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.

OK, this matches my understanding. Good we are on the same page.

> 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.

But this is still not clear to me. __PageMovable pages are isolated by
isolate_movable_page and then jump to isolate_succes. Does that somehow
changes the nature of the page being compound or tat thp_nr_page would
start working on those pages.
-- 
Michal Hocko
SUSE Labs