Re: [v2 PATCH] mm: migrate: handle freed page at the first place

[Michal Hocko]

On Fri 15-11-19 02:24:29, Yang Shi wrote:
> When doing migration if the freed page is met, we just return without
> migrating it since it is pointless to migrate a freed page.  But, the
> current code allocates target page unconditionally before handling freed
> page, if the page is freed, the newly allocated will be just freed.  It
> doesn't make too much sense and is just a waste of time although
> migrating freed page is rare.
> 
> So, handle freed page at the before that to avoid unnecessary page
> allocation and free.
> 
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>

I would be really surprised if this led to any runtime visible effect
but I do agree that one less put_page path looks slightly better. For
that reason
Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
> v2: * Keep thp migration support check before handling freed page per Michal Hocko
>     * Fixed the build warning reported by 0-day
> 
>  mm/migrate.c | 14 +++++---------
>  1 file changed, 5 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 4fe45d1..a8f87cb 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1168,15 +1168,11 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
>  				   enum migrate_reason reason)
>  {
>  	int rc = MIGRATEPAGE_SUCCESS;
> -	struct page *newpage;
> +	struct page *newpage = NULL;
>  
>  	if (!thp_migration_supported() && PageTransHuge(page))
>  		return -ENOMEM;
>  
> -	newpage = get_new_page(page, private);
> -	if (!newpage)
> -		return -ENOMEM;
> -
>  	if (page_count(page) == 1) {
>  		/* page was freed from under us. So we are done. */
>  		ClearPageActive(page);
> @@ -1187,13 +1183,13 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
>  				__ClearPageIsolated(page);
>  			unlock_page(page);
>  		}
> -		if (put_new_page)
> -			put_new_page(newpage, private);
> -		else
> -			put_page(newpage);
>  		goto out;
>  	}
>  
> +	newpage = get_new_page(page, private);
> +	if (!newpage)
> +		return -ENOMEM;
> +
>  	rc = __unmap_and_move(page, newpage, force, mode);
>  	if (rc == MIGRATEPAGE_SUCCESS)
>  		set_page_owner_migrate_reason(newpage, reason);
> -- 
> 1.8.3.1
> 

-- 
Michal Hocko
SUSE Labs