Re: [PATCH 2/4] mm, migrate: allocation new page lazyily in unmap_and_move()

Johannes Weiner
2013-08-02T19:41:13Z

On Fri, Aug 02, 2013 at 11:07:57AM +0900, Joonsoo Kim wrote:
> We don't need a new page and then go out immediately if some condition
> is met. Allocation has overhead in comparison with some condition check,
> so allocating lazyily is preferable solution.
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 6f0c244..86db87e 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -864,10 +864,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
> {
> int rc = 0;
> int *result = NULL;
> - struct page *newpage = get_new_page(page, private, &result);
> -
> - if (!newpage)
> - return -ENOMEM;
> + struct page *newpage = NULL;
>
> if (page_count(page) == 1) {
> /* page was freed from under us. So we are done. */
> @@ -878,6 +875,10 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
> if (unlikely(split_huge_page(page)))
> goto out;
>
> + newpage = get_new_page(page, private, &result);
> + if (!newpage)
> + return -ENOMEM;

get_new_page() sets up result to communicate error codes from the
following checks. While the existing ones (page freed and thp split
failed) don't change rc, somebody else might add a condition whose
error code should be propagated back into *result but miss it.

Please leave get_new_page() where it is. The win from this change is
not big enough to risk these problems.

Thanks!