Re: [PATCH v3 7/9] mm, hugetlb: add VM_NORESERVE check in vma_has_reserves()

[Hillf Danton <dhillf@gmail.com>]

On Mon, Jul 29, 2013 at 1:28 PM, Joonsoo Kim <iamjoonsoo.kim@lge.com> wrote:
> 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.
>
> HugePages_Free:       99
> HugePages_Rsvd:      100
>
> Number of free should be higher or equal than number of reserve,
> but this aren't. This represent that non reserved shared mapping steal
> a reserved page. Non reserved shared mapping should not eat into
> reserve space.
>
> If we consider VM_NORESERVE in vma_has_reserve() and return 0 which mean
> that we don't have reserved pages, then we check that we have enough
> free pages in dequeue_huge_page_vma(). This prevent to steal
> a reserved page.
>
> With this change, above test generate a SIGBUG which is correct,
> because all free pages are reserved and non reserved shared mapping
> can't get a free page.
>
> Reviewed-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
> Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>
Acked-by: Hillf Danton <dhillf@gmail.com>

> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 1f6b3a6..ca15854 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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