[patch] possible-cause-of-a-page_mapped-bug fix (upstream)

mm-fix-possible-cause-of-a-page_mapped-bug.patch (text/plain), 1.62 KB, created by RumpletonBongworth on 2011-03-19 04:33:23 UTC

From a3e8cc643d22d2c8ed36b9be7d9c9ca21efcf7f7 Mon Sep 17 00:00:00 2001
From: Hugh Dickins <[e-mail]>
Date: Wed, 23 Feb 2011 21:39:49 -0800
Subject: [PATCH] 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 <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>
---
 mm/mremap.c |    4 +---
 1 files changed, 1 insertions(+), 3 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index 9925b63..1de98d4 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;
 	}
 
 	/*
-- 
1.7.4.1