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

[Michal Hocko]

On Tue 12-11-19 06:09:25, 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 did two things before handling freed page:
> 
> 1. Return -ENOMEM if the page is THP and THP migration is not supported.
> 2. Allocate target page unconditionally.
> 
> Both makes not too much sense.  If we handle freed page at the first place
> we don't have to worry about allocating/freeing target page and split
> THP at all.
> 
> For example (worst case) if we are trying to migrate a freed THP without
> THP migration supported, the migrate_pages() would just split the THP then
> retry to migrate base pages one by one by pointless allocating and freeing
> pages, this is just waste of time.
> 
> I didn't run into any actual problem with the current code (or I may
> just not notice it yet), it was found by visual inspection.

It would be preferable to accompany a change like this with some actual
numbers. A race with page freeing should be a very rare situation. Maybe
it is not under some workloads but that would better be checked and
documented. I also do not like to do page state changes for THP
migration without a support. I cannot really say this is 100% correct
from top of my head and I do not see a sufficient justification to go
and chase all those tiny details because that is time consuming.

> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> ---
>  mm/migrate.c | 18 +++++++-----------
>  1 file changed, 7 insertions(+), 11 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 4fe45d1..ef96997 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1170,13 +1170,6 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
>  	int rc = MIGRATEPAGE_SUCCESS;
>  	struct page *newpage;
>  
> -	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 +1180,16 @@ 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;
>  	}
>  
> +	if (!thp_migration_supported() && PageTransHuge(page))
> +		return -ENOMEM;
> +
> +	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