linux-kernel - Re: [PATCH 7/9] mm, hugetlb: add VM_NORESERVE check in vma_has

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Message-ID: <87sizfj2v5.fsf@linux.vnet.ibm.com>
Date:	Mon, 15 Jul 2013 20:18:30 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Michal Hocko <mhocko@...e.cz>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hugh Dickins <hughd@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	David Gibson <david@...son.dropbear.id.au>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Joonsoo Kim <js1304@...il.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH 7/9] mm, hugetlb: add VM_NORESERVE check in vma_has_reserves()

Joonsoo Kim <iamjoonsoo.kim@....com> writes:

> If we map the region with MAP_NORESERVE and MAP_SHARED,
> we can skip to check reserve counting and eventually we cannot be ensured
> to allocate a huge page in fault time.
> With following example code, you can easily find this situation.
>
> Assume 2MB, nr_hugepages = 100
>
>         fd = hugetlbfs_unlinked_fd();
>         if (fd < 0)
>                 return 1;
>
>         size = 200 * MB;
>         flag = MAP_SHARED;
>         p = mmap(NULL, size, PROT_READ|PROT_WRITE, flag, fd, 0);
>         if (p == MAP_FAILED) {
>                 fprintf(stderr, "mmap() failed: %s\n", strerror(errno));
>                 return -1;
>         }
>
>         size = 2 * MB;
>         flag = MAP_ANONYMOUS | MAP_SHARED | MAP_HUGETLB | MAP_NORESERVE;
>         p = mmap(NULL, size, PROT_READ|PROT_WRITE, flag, -1, 0);
>         if (p == MAP_FAILED) {
>                 fprintf(stderr, "mmap() failed: %s\n", strerror(errno));
>         }
>         p[0] = '0';
>         sleep(10);
>
> During executing sleep(10), run 'cat /proc/meminfo' on another process.
> You'll find a mentioned problem.
>
> Solution is simple. We should check VM_NORESERVE in vma_has_reserves().
> This prevent to use a pre-allocated huge page if free count is under
> the reserve count.
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>

May be it is better to say
"non reserved shared mapping should not eat into reserve space. So
return error when we don't find enough free space."


>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 6c1eb9b..f6a7a4e 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -464,6 +464,8 @@ void reset_vma_resv_huge_pages(struct vm_area_struct *vma)
>  /* Returns true if the VMA has associated reserve pages */
>  static int vma_has_reserves(struct vm_area_struct *vma)
>  {
> +	if (vma->vm_flags & VM_NORESERVE)
> +		return 0;
>  	if (vma->vm_flags & VM_MAYSHARE)
>  		return 1;
>  	if (is_vma_resv_set(vma, HPAGE_RESV_OWNER))
> -- 
> 1.7.9.5

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