linux-kernel - Re: [PATCH v3] mm/swap: Fix release_pages() when releasing devmap pages

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Date:   Tue, 4 Jun 2019 12:48:18 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     <ira.weiny@...el.com>, Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v3] mm/swap: Fix release_pages() when releasing devmap
 pages

On 6/4/19 9:48 AM, ira.weiny@...el.com wrote:
> From: Ira Weiny <ira.weiny@...el.com>
> 
> release_pages() is an optimized version of a loop around put_page().
> Unfortunately for devmap pages the logic is not entirely correct in
> release_pages().  This is because device pages can be more than type
> MEMORY_DEVICE_PUBLIC.  There are in fact 4 types, private, public, FS
> DAX, and PCI P2PDMA.  Some of these have specific needs to "put" the
> page while others do not.
> 
> This logic to handle any special needs is contained in
> put_devmap_managed_page().  Therefore all devmap pages should be
> processed by this function where we can contain the correct logic for a
> page put.
> 
> Handle all device type pages within release_pages() by calling
> put_devmap_managed_page() on all devmap pages.  If
> put_devmap_managed_page() returns true the page has been put and we
> continue with the next page.  A false return of
> put_devmap_managed_page() means the page did not require special
> processing and should fall to "normal" processing.
> 
> This was found via code inspection while determining if release_pages()
> and the new put_user_pages() could be interchangeable.[1]
> 
> [1] https://lore.kernel.org/lkml/20190523172852.GA27175@iweiny-DESK2.sc.intel.com/
> 
> Cc: Jérôme Glisse <jglisse@...hat.com>
> Cc: Michal Hocko <mhocko@...e.com>
> Reviewed-by: Dan Williams <dan.j.williams@...el.com>
> Reviewed-by: John Hubbard <jhubbard@...dia.com>
> Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> 
> ---
> Changes from V2:
> 	Update changelog for more clarity as requested by Michal
> 	Update comment WRT "failing" of put_devmap_managed_page()
> 
> Changes from V1:
> 	Add comment clarifying that put_devmap_managed_page() can still
> 	fail.
> 	Add Reviewed-by tags.
> 
>  mm/swap.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/swap.c b/mm/swap.c
> index 7ede3eddc12a..6d153ce4cb8c 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -740,15 +740,20 @@ 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;
> +			/*
> +			 * Not all zone-device-pages require special
> +			 * processing.  Those pages return 'false' from
> +			 * put_devmap_managed_page() expecting a call to
> +			 * put_page_testzero()
> +			 */

Just a documentation tweak: how about: 

			/*
			 * ZONE_DEVICE pages that return 'false' from 
			 * put_devmap_managed_page() do not require special 
			 * processing, and instead, expect a call to 
			 * put_page_testzero().
			 */


thanks,
-- 
John Hubbard
NVIDIA

> +			if (put_devmap_managed_page(page))
> +				continue;
>  		}
>  
>  		page = compound_head(page);
>