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

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 26 Jun 2019 21:10:00 +0800 Pingfan Liu <kernelfans@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?