hugetlb: acquire the i_mmap_lock before walking the prio_tree to unmap a page

When the owner of a mapping fails COW because a child process is holding a
reference, the children VMAs are walked and the page is unmapped.  The
i_mmap_lock is taken for the unmapping of the page but not the walking of
the prio_tree.  In theory, that tree could be changing if the lock is not
held.  This patch takes the i_mmap_lock properly for the duration of the
prio_tree walk.

[hugh.dickins@tiscali.co.uk: Spotted the problem in the first place]
Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Acked-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 1 deletions

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 2ef66a2a148d..6df8065039eb 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2237,6 +2237,12 @@ static int unmap_ref_private(struct mm_struct *mm, struct vm_area_struct *vma,
                 + (vma->vm_pgoff >> PAGE_SHIFT);
         mapping = (struct address_space *)page_private(page);
 
+        /*
+         * Take the mapping lock for the duration of the table walk. As
+         * this mapping should be shared between all the VMAs,
+         * __unmap_hugepage_range() is called as the lock is already held
+         */
+        spin_lock(&mapping->i_mmap_lock);
         vma_prio_tree_foreach(iter_vma, &iter, &mapping->i_mmap, pgoff, pgoff) {
                 /* Do not unmap the current VMA */
                 if (iter_vma == vma)
@@ -2250,10 +2256,11 @@ static int unmap_ref_private(struct mm_struct *mm, struct vm_area_struct *vma,
                  * from the time of fork. This would look like data corruption
                  */
                 if (!is_vma_resv_set(iter_vma, HPAGE_RESV_OWNER))
-                        unmap_hugepage_range(iter_vma,
+                        __unmap_hugepage_range(iter_vma,
                                 address, address + huge_page_size(h),
                                 page);
         }
+        spin_unlock(&mapping->i_mmap_lock);
 
         return 1;
 }