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

[Andrew Morton]

On Tue, 12 Nov 2019 06:09:25 +0800 Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> 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.
> 
>
> --- 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) {

Is it possible to have (!thp_migration_supported() &&
PageTransHuge(page) && page_count(page) == 1)?  If so, isn't this new
behviour?

>  		/* 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);