Re: [PATCH v2] mm/swap: Fix release_pages() when releasing devmap pages

[Michal Hocko]

On Fri 24-05-19 10:36:56, ira.weiny@xxxxxxxxx wrote:
> From: Ira Weiny <ira.weiny@xxxxxxxxx>
> 
> Device pages can be more than type MEMORY_DEVICE_PUBLIC.
> 
> Handle all device pages within release_pages()
> 
> This was found via code inspection while determining if release_pages()
> and the new put_user_pages() could be interchangeable.

Please expand more about who is such a user and why does it use
release_pages rather than put_*page API. The above changelog doesn't
really help understanding what is the actual problem. I also do not
understand the fix and a failure mode from release_pages is just scary.
It is basically impossible to handle the error case. So what is going on
here?

> Cc: Jérôme Glisse <jglisse@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
> Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> 
> ---
> Changes from V1:
> 	Add comment clarifying that put_devmap_managed_page() can still
> 	fail.
> 	Add Reviewed-by tags.
> 
>  mm/swap.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/swap.c b/mm/swap.c
> index 9d0432baddb0..f03b7b4bfb4f 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -740,15 +740,18 @@ void release_pages(struct page **pages, int nr)
>  		if (is_huge_zero_page(page))
>  			continue;
>  
> -		/* Device public page can not be huge page */
> -		if (is_device_public_page(page)) {
> +		if (is_zone_device_page(page)) {
>  			if (locked_pgdat) {
>  				spin_unlock_irqrestore(&locked_pgdat->lru_lock,
>  						       flags);
>  				locked_pgdat = NULL;
>  			}
> -			put_devmap_managed_page(page);
> -			continue;
> +			/*
> +			 * zone-device-pages can still fail here and will
> +			 * therefore need put_page_testzero()
> +			 */
> +			if (put_devmap_managed_page(page))
> +				continue;
>  		}
>  
>  		page = compound_head(page);
> -- 
> 2.20.1
> 

-- 
Michal Hocko
SUSE Labs