Re: [PATCH 1/2] [PATCH] Fix a hugepage reservation check for MAP_SHARED

[Andy Whitcroft <apw@shadowen.org>]

On Thu, Jul 10, 2008 at 06:30:21PM +0100, Mel Gorman wrote:
> 
> When removing a huge page from the hugepage pool for a fault the system
> checks to see if the mapping requires additional pages to be reserved, and
> if it does whether there are any unreserved pages remaining.  If not, the
> allocation fails without even attempting to get a page. In order to determine
> whether to apply this check we call vma_has_private_reserves() which tells us
> if this vma is MAP_PRIVATE and is the owner.  This incorrectly triggers the
> remaining reservation test for MAP_SHARED mappings which prevents allocation
> of the final page in the pool even though it is reserved for this mapping.
> 
> In reality we only want to check this for MAP_PRIVATE mappings where the
> process is not the original mapper.  Replace vma_has_private_reserves() with
> vma_has_reserves() which indicates whether further reserves are required,
> and update the caller.

Acked-by: Andy Whitcroft <apw@shadowen.org>

> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> ---
> 
>  mm/hugetlb.c |   12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff -rup -X /usr/src/patchset-0.6/bin//dontdiff linux-2.6.26-rc8-mm1-clean/mm/hugetlb.c linux-2.6.26-rc8-mm1-fix-needsreserve-check/mm/hugetlb.c
> --- linux-2.6.26-rc8-mm1-clean/mm/hugetlb.c	2008-07-08 11:54:34.000000000 -0700
> +++ linux-2.6.26-rc8-mm1-fix-needsreserve-check/mm/hugetlb.c	2008-07-08 12:41:36.000000000 -0700
> @@ -343,13 +343,13 @@ void reset_vma_resv_huge_pages(struct vm
>  }
>  
>  /* Returns true if the VMA has associated reserve pages */
> -static int vma_has_private_reserves(struct vm_area_struct *vma)
> +static int vma_has_reserves(struct vm_area_struct *vma)
>  {
>  	if (vma->vm_flags & VM_SHARED)
> -		return 0;
> -	if (!is_vma_resv_set(vma, HPAGE_RESV_OWNER))
> -		return 0;
> -	return 1;
> +		return 1;
> +	if (is_vma_resv_set(vma, HPAGE_RESV_OWNER))
> +		return 1;
> +	return 0;
>  }
>  
>  static void clear_huge_page(struct page *page,
> @@ -421,7 +421,7 @@ static struct page *dequeue_huge_page_vm
>  	 * have no page reserves. This check ensures that reservations are
>  	 * not "stolen". The child may still get SIGKILLed
>  	 */
> -	if (!vma_has_private_reserves(vma) &&
> +	if (!vma_has_reserves(vma) &&
>  			h->free_huge_pages - h->resv_huge_pages == 0)
>  		return NULL;
>  

-apw