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Subject[PATCH 7/9] mm, hugetlb: add VM_NORESERVE check in vma_has_reserves()
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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@lge.com>

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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