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Subject[PATCH v3 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.

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>

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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