From 73933b3315e65d92e0c6cc4f8b4c51c9dc87546b Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Date: Wed, 24 Jun 2015 16:56:30 -0700
Subject: [PATCH] mm: drop bogus VM_BUG_ON_PAGE assert in put_page() codepath
My commit 8d63d99a5dfb ("mm: avoid tail page refcounting on non-THP
compound pages") which was merged during 4.1 merge window caused
regression:
page:ffffea0010a15040 count:0 mapcount:1 mapping: (null) index:0x0
flags: 0x8000000000008014(referenced|dirty|tail)
page dumped because: VM_BUG_ON_PAGE(page_mapcount(page) != 0)
------------[ cut here ]------------
kernel BUG at mm/swap.c:134!
The problem can be reproduced by playing *two* audio files at the same
time and then stopping one of players. I used two mplayers to trigger
this.
The VM_BUG_ON_PAGE() which triggers the bug is bogus:
Sound subsystem uses compound pages for its buffers, but unlike most
__GFP_COMP sound maps compound pages to userspace with PTEs.
In our case with two players map the buffer twice and therefore elevates
page_mapcount() on tail pages by two. When one of players exits it
unmaps the VMA and drops page_mapcount() to one and try to release
reference on the page with put_page().
My commit changes which path it takes under put_compound_page(). It hits
put_unrefcounted_compound_page() where VM_BUG_ON_PAGE() is. It sees
page_mapcount() == 1. The function wrongly assumes that subpages of
compound page cannot be be mapped by itself with PTEs..
The solution is simply drop the VM_BUG_ON_PAGE().
Note: there's no need to move the check under put_page_testzero().
Allocator will check the mapcount by itself before putting on free list.
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: Andrea Arcangeli <aarcange@redhat.com>
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
Reported-by: Borislav Petkov <bp@alien8.de>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
mm/swap.c | 1 -
1 file changed, 1 deletion(-)
diff --git a/mm/swap.c b/mm/swap.c
index a7251a8ed5329..a3a0a2f1f7c3d 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -131,7 +131,6 @@ void put_unrefcounted_compound_page(struct page *page_head, struct page *page)
* here, see the comment above this function.
*/
VM_BUG_ON_PAGE(!PageHead(page_head), page_head);
- VM_BUG_ON_PAGE(page_mapcount(page) != 0, page);
if (put_page_testzero(page_head)) {
/*
* If this is the tail of a slab THP page,
--
2.39.5