On Mon, Jul 15, 2013 at 06:52:45PM +0900, Joonsoo Kim 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.
>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.
>
Reviewed-by: Wanpeng Li <liwanp@xxxxxxxxxxxxxxxxxx>
>Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
>
>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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