linux-kernel - Re: [RFC PATCH 1/4] mm: Have the mempolicy pagewalk to avoid positive callback return codes

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Date:   Fri, 11 Oct 2019 16:08:40 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        torvalds@...ux-foundation.org,
        Thomas Hellstrom <thellstrom@...are.com>
Subject: Re: [RFC PATCH 1/4] mm: Have the mempolicy pagewalk to avoid
 positive callback return codes

On Thu, Oct 10, 2019 at 03:40:55PM +0200, Thomas Hellström (VMware) wrote:
> From: Linus Torvalds <torvalds@...ux-foundation.org>
> 
> The pagewalk code is being reworked to have positive callback return codes
> do walk control. Avoid using positive return codes: "1" is replaced by
> "-EBUSY".
> 
> Co-developed-by: Thomas Hellstrom <thellstrom@...are.com>
> Signed-off-by: Thomas Hellstrom <thellstrom@...are.com>
> ---
>  mm/mempolicy.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 4ae967bcf954..df34c7498c27 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -482,8 +482,8 @@ static int queue_pages_pmd(pmd_t *pmd, spinlock_t *ptl, unsigned long addr,
>   *
>   * queue_pages_pte_range() has three possible return values:
>   * 0 - pages are placed on the right node or queued successfully.
> - * 1 - there is unmovable page, and MPOL_MF_MOVE* & MPOL_MF_STRICT were
> - *     specified.
> + * -EBUSY - there is unmovable page, and MPOL_MF_MOVE* & MPOL_MF_STRICT were
> + *          specified.
>   * -EIO - only MPOL_MF_STRICT was specified and an existing page was already
>   *        on a node that does not follow the policy.
>   */
> @@ -503,7 +503,7 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
>  	if (ptl) {
>  		ret = queue_pages_pmd(pmd, ptl, addr, end, walk);
>  		if (ret != 2)
> -			return ret;
> +			return (ret == 1) ? -EBUSY : ret;

It would be cleaner to propagate the error code logic to queue_pages_pmd()
too: 0 - placed, 1 - split, -EBUSY - unmovable, ...

>  	}
>  	/* THP was split, fall through to pte walk */
>  
> @@ -546,7 +546,7 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
>  	cond_resched();
>  
>  	if (has_unmovable)
> -		return 1;
> +		return -EBUSY;
>  
>  	return addr != end ? -EIO : 0;
>  }
> @@ -669,9 +669,9 @@ static const struct mm_walk_ops queue_pages_walk_ops = {
>   * passed via @private.
>   *
>   * queue_pages_range() has three possible return values:
> - * 1 - there is unmovable page, but MPOL_MF_MOVE* & MPOL_MF_STRICT were
> - *     specified.
>   * 0 - queue pages successfully or no misplaced page.
> + * -EBUSY - there is unmovable page, but MPOL_MF_MOVE* & MPOL_MF_STRICT were
> + *	    specified.
>   * -EIO - there is misplaced page and only MPOL_MF_STRICT was specified.
>   */
>  static int
> @@ -1285,7 +1285,7 @@ static long do_mbind(unsigned long start, unsigned long len,
>  	ret = queue_pages_range(mm, start, end, nmask,
>  			  flags | MPOL_MF_INVERT, &pagelist);
>  
> -	if (ret < 0) {
> +	if (ret < 0 && ret != -EBUSY) {
>  		err = -EIO;
>  		goto up_out;
>  	}
> @@ -1303,7 +1303,7 @@ static long do_mbind(unsigned long start, unsigned long len,
>  				putback_movable_pages(&pagelist);
>  		}
>  
> -		if ((ret > 0) || (nr_failed && (flags & MPOL_MF_STRICT)))
> +		if ((ret < 0) || (nr_failed && (flags & MPOL_MF_STRICT)))
>  			err = -EIO;
>  	} else
>  		putback_movable_pages(&pagelist);
> -- 
> 2.21.0
> 

-- 
 Kirill A. Shutemov