[RFC PATCH] mm: WARN_ON_ONCE if f_op->mmap() change vma's start address

[Joonsoo Kim]

During reviewing the source code, I found a comment which mention that
after f_op->mmap(), vma's start address can be changed.
I didn't verify that it is really possible, because there are so many
f_op->mmap() implementation. But if there are some mmap() which change
vma's start address, it is possible error situation, because we already
prepare prev vma, rb_link and rb_parent and these are related to original
address.

So add WARN_ON_ONCE for finding that this situtation really happens.

Signed-off-by: Joonsoo Kim <js1304@xxxxxxxxx>

diff --git a/mm/mmap.c b/mm/mmap.c
index 2d94235..36567b7 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1333,7 +1333,11 @@ munmap_back:
 		 *
 		 * Answer: Yes, several device drivers can do it in their
 		 *         f_op->mmap method. -DaveM
+		 * Bug: If addr is changed, prev, rb_link, rb_parent should
+		 *      be updated for vma_link()
 		 */
+		WARN_ON_ONCE(addr != vma->vm_start);
+
 		addr = vma->vm_start;
 		pgoff = vma->vm_pgoff;
 		vm_flags = vma->vm_flags;
-- 
1.7.9.5

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