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    SubjectRe: [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@nvidia.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@nvidia.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

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