linux-kernel - [PATCH] mm: fix possible cause of a page

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Message-ID: <alpine.LSU.2.00.1102232136020.2239@sister.anvils>
Date:	Wed, 23 Feb 2011 21:39:49 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Robert Swiecki <robert@...ecki.net>,
	Miklos Szeredi <miklos@...redi.hu>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
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 <hughd@...gle.com>
---

 mm/mremap.c |    4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

--- 2.6.38-rc6/mm/mremap.c	2011-01-18 22:04:56.000000000 -0800
+++ linux/mm/mremap.c	2011-02-23 15:29:52.000000000 -0800
@@ -94,9 +94,7 @@ static void move_ptes(struct vm_area_str
 		 */
 		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;
 	}

 	/*
--
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