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

[Johannes Weiner <hannes@cmpxchg.org>]

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!

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