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

[Hugh Dickins <hughd@google.com>]

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 5f70b962ccc2 "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>
---
 mm/rmap.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- linux.orig/mm/rmap.c	2011-05-27 19:05:27.000000000 -0700
+++ linux/mm/rmap.c	2011-05-27 20:07:44.601361236 -0700
@@ -1014,7 +1014,7 @@ void do_page_add_anon_rmap(struct 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
 
 	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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