[34-longterm 154/247] mm: fix possible cause of a page_mapped BUG

Paul Gortmaker
2011-06-23T17:56:43Z

From: Hugh Dickins <hughd@google.com>

-------------------
This is a commit scheduled for the next v2.6.34 longterm release.
If you see a problem with using this for longterm, please comment.
-------------------

commit a3e8cc643d22d2c8ed36b9be7d9c9ca21efcf7f7 upstream.

Robert Swiecki reported a BUG_ON(page_mapped) from a fuzzer, punching
a hole with madvise(,, MADV_REMOVE). That path is under mutex, and
cannot be explained by lack of serialization in unmap_mapping_range().

Reviewing the code, I found one place where vm_truncate_count handling
should have been updated, when I switched at the last minute from one
way of managing the restart_addr to another: mremap move changes the
virtual addresses, so it ought to adjust the restart_addr.

But rather than exporting the notion of restart_addr from memory.c, or
converting to restart_pgoff throughout, simply reset vm_truncate_count
to 0 to force a rescan if mremap move races with preempted truncation.

We have no confirmation that this fixes Robert's BUG,
but it is a fix that's worth making anyway.

Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
---
mm/mremap.c | 4 +---
1 files changed, 1 insertions(+), 3 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index cde56ee..10d5f62 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -91,9 +91,7 @@ static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
*/
mapping = vma->vm_file->f_mapping;
spin_lock(&mapping->i_mmap_lock);
- if (new_vma->vm_truncate_count &&
- new_vma->vm_truncate_count != vma->vm_truncate_count)
- new_vma->vm_truncate_count = 0;
+ new_vma->vm_truncate_count = 0;
}

/*
--
1.7.4.4