On Mon, Jul 29, 2013 at 1:28 PM, Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
> Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
>
Acked-by: Hillf Danton <dhillf@xxxxxxxxx>
> 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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