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
>
