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

[Yang Shi <shy828301@gmail.com>]

On Fri, Oct 30, 2020 at 6:36 AM Michal Hocko <mhocko@suse.com> 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?

I have the same question, can they be compound? If they can be
compound, PageTransHuge() can't tell from THP and compound movable
page, right?

>
> --
> Michal Hocko
> SUSE Labs
>