Commit 5dbe0af4 authored May 28, 2011 by Hugh Dickins Committed by Linus Torvalds May 28, 2011

mm: fix kernel BUG at mm/rmap.c:1017!

I've hit the "address >= vma->vm_end" check in do_page_add_anon_rmap()
just once. The stack showed khugepaged allocation trying to compact
pages: the call to page_add_anon_rmap() coming from remove_migration_pte().

That path holds anon_vma lock, but does not hold mmap_sem: it can
therefore race with a split_vma(), and in commit 5f70b962 "mmap:
avoid unnecessary anon_vma lock" we just took away the anon_vma lock
protection when adjusting vma->vm_end.

I don't think that particular BUG_ON ever caught anything interesting,
so better replace it by a comment, than reinstate the anon_vma locking.

Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 826267cf

mm/rmap.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -1014,7 +1014,7 @@ void do_page_add_anon_rmap(struct page *page,
		return;

		VM_BUG_ON(!PageLocked(page));
		VM_BUG_ON(address < vma->vm_start \|\| address >= vma->vm_end);
		/* address might be in next vma when migration races vma_adjust */
		if (first)
		__page_set_anon_rmap(page, vma, address, exclusive);
		else
		@@ -1709,7 +1709,7 @@ void hugepage_add_anon_rmap(struct page *page,

		BUG_ON(!PageLocked(page));
		BUG_ON(!anon_vma);
		BUG_ON(address < vma->vm_start \|\| address >= vma->vm_end);
		/* address might be in next vma when migration races vma_adjust */
		first = atomic_inc_and_test(&page->_mapcount);
		if (first)
		__hugepage_set_anon_rmap(page, vma, address, 0);

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