Commit a734bcc8 authored Jan 10, 2012 by Hillf Danton Committed by Linus Torvalds Jan 10, 2012

hugetlb: detect race upon page allocation failure during COW



Currently we are not rechecking pte_same in hugetlb_cow after we take ptl
lock again in the page allocation failure code path and simply retry
again.  This is not an issue at the moment because hugetlb fault path is
protected by hugetlb_instantiation_mutex so we cannot race.

The original page is locked and so we cannot race even with the page
migration.

Let's add the pte_same check anyway as we want to be consistent with the
other check later in this function and be safe if we ever remove the
mutex.

[mhocko@suse.cz: reworded the changelog]
Signed-off-by: Hillf Danton <dhillf@gmail.com>
Signed-off-by: Michal Hocko <mhocko@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Johannes Weiner <jweiner@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 5f8aefd4

mm/hugetlb.c

+8 −1

Original line number	Diff line number	Diff line
		@@ -2408,7 +2408,14 @@ static int hugetlb_cow(struct mm_struct mm, struct vm_area_struct vma,
		BUG_ON(page_count(old_page) != 1);
		BUG_ON(huge_pte_none(pte));
		spin_lock(&mm->page_table_lock);
		ptep = huge_pte_offset(mm, address & huge_page_mask(h));
		if (likely(pte_same(huge_ptep_get(ptep), pte)))
		goto retry_avoidcopy;
		/*
		* race occurs while re-acquiring page_table_lock, and
		* our job is done.
		*/
		return 0;
		}
		WARN_ON_ONCE(1);
		}