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

[Vlastimil Babka]

On 10/30/20 7:55 PM, Yang Shi wrote:
> On Fri, Oct 30, 2020 at 11:39 AM Zi Yan <ziy@xxxxxxxxxx> wrote:
> > On 30 Oct 2020, at 14:33, Yang Shi wrote:
> >
> > > On Fri, Oct 30, 2020 at 6:36 AM Michal Hocko <mhocko@xxxxxxxx> 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@xxxxxxxxxx>
> > >>>>>
> > >>>>> 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@xxxxxxxxxx>
> > >>>>> ---
> > >>>>>  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?
> >
> > Right. I have updated the patch and use compound_nr instead.
>
> Thanks. Actually I'm wondering what kind of movable page could be
> compound. Any real examples?

Looks like there's currently none. Compaction also wouldn't work properly with 
movable pages with order>0 as the free page scanner looks for order-0 pages 
only. But it won't hurt to use compound_nr() anyway.

> > —
> > Best Regards,
> > Yan Zi