On Wed, 26 Jun 2019 21:10:00 +0800 Pingfan Liu <kernelf...@gmail.com> 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.
> + 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.
Could we please get a comment in here whcih fully explains the logic
and any assumptions?
