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

[Aneesh Kumar K.V]

Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> 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@xxxxxxx>

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>

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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