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

[Yang Shi]

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?

>
> —
> Best Regards,
> Yan Zi