From: Hugh Dickins <hughd@google.com>
Date: Thu, 24 Feb 2011 05:39:49 +0000 (-0800)
Subject: mm: fix possible cause of a page_mapped BUG
X-Git-Tag: v2.6.38-rc7~44
X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=a3e8cc643d22d2c8ed36b9be7d9c9ca21efcf7f7;p=users%2Fjedix%2Flinux-maple.git

mm: fix possible cause of a page_mapped BUG

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>
---

diff --git a/mm/mremap.c b/mm/mremap.c
index 9925b6391b80..1de98d492ddc 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -94,9 +94,7 @@ 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;
 	}
 
 	/*