Re: [PATCHv4] mm/gup: speed up check_and_migrate_cma_pages() on huge page

[Pingfan Liu]

On Thu, Jun 27, 2019 at 7:15 AM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, 26 Jun 2019 21:10:00 +0800 Pingfan Liu <kernelfans@xxxxxxxxx> wrote:
>
> > Both hugetlb and thp locate on the same migration type of pageblock, since
> > they are allocated from a free_list[]. Based on this fact, it is enough to
> > check on a single subpage to decide the migration type of the whole huge
> > page. By this way, it saves (2M/4K - 1) times loop for pmd_huge on x86,
> > similar on other archs.
> >
> > Furthermore, when executing isolate_huge_page(), it avoid taking global
> > hugetlb_lock many times, and meanless remove/add to the local link list
> > cma_page_list.
> >
> > ...
> >
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -1342,19 +1342,22 @@ static long check_and_migrate_cma_pages(struct task_struct *tsk,
> >       LIST_HEAD(cma_page_list);
> >
> >  check_again:
> > -     for (i = 0; i < nr_pages; i++) {
> > +     for (i = 0; i < nr_pages;) {
> > +
> > +             struct page *head = compound_head(pages[i]);
> > +             long step = 1;
> > +
> > +             if (PageCompound(head))
>
> I suspect this would work correctly if the PageCompound test was simply
> removed.  Not that I'm really suggesting that it be removed - dunno.
Yes, you are right. compound_order() can safely run on normal page,
which means we can drop the check PageCompound().

>
> > +                     step = (1 << compound_order(head)) - (pages[i] - head);
>
> I don't understand this statement.  Why does the position of this page
> in the pages[] array affect anything?  There's an assumption about the
> contents of the skipped pages, I assume.
Because gup may start from a tail page.
>
> Could we please get a comment in here whcih fully explains the logic
> and any assumptions?
Sure, I will.

Thanks,
  Pingfan
>