Re: [PATCH 1/1] mm/migrate: fix list corruption in migration of non-LRU movable pages

[Michal Hocko]

On Tue 03-09-19 16:27:46, sunqiuyang wrote:
> From: Qiuyang Sun <sunqiuyang@xxxxxxxxxx>
> 
> Currently, after a page is migrated, it
> 1) has its PG_isolated flag cleared in move_to_new_page(), and
> 2) is deleted from its LRU list (cc->migratepages) in unmap_and_move().
> However, between steps 1) and 2), the page could be isolated by another
> thread in isolate_movable_page(), and added to another LRU list, leading
> to list_del corruption later.

Care to explain the race? Both paths use page_lock AFAICS
> 
> This patch fixes the bug by moving list_del into the critical section
> protected by lock_page(), so that a page will not be isolated again before
> it has been deleted from its LRU list.
> 
> Signed-off-by: Qiuyang Sun <sunqiuyang@xxxxxxxxxx>
> ---
>  mm/migrate.c | 11 +++--------
>  1 file changed, 3 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index a42858d..c58a606 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1124,6 +1124,8 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
>  	/* Drop an anon_vma reference if we took one */
>  	if (anon_vma)
>  		put_anon_vma(anon_vma);
> +	if (rc != -EAGAIN)
> +		list_del(&page->lru);
>  	unlock_page(page);
>  out:
>  	/*
> @@ -1190,6 +1192,7 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
>  			put_new_page(newpage, private);
>  		else
>  			put_page(newpage);
> +		list_del(&page->lru);
>  		goto out;
>  	}
>  
> @@ -1200,14 +1203,6 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
>  out:
>  	if (rc != -EAGAIN) {
>  		/*
> -		 * A page that has been migrated has all references
> -		 * removed and will be freed. A page that has not been
> -		 * migrated will have kepts its references and be
> -		 * restored.
> -		 */
> -		list_del(&page->lru);
> -
> -		/*
>  		 * Compaction can migrate also non-LRU pages which are
>  		 * not accounted to NR_ISOLATED_*. They can be recognized
>  		 * as __PageMovable
> -- 
> 1.8.3.1

-- 
Michal Hocko
SUSE Labs