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authorKirill A. Shutemov <kirill.shutemov@linux.intel.com>2015-06-24 16:56:30 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2015-06-24 17:49:42 -0700
commit73933b3315e65d92e0c6cc4f8b4c51c9dc87546b (patch)
tree99b3553a574d3ab4752e02a192d5c46f6aadd3c4
parenta9919c79359df9a706ff9e14fc0a93cd15791c9b (diff)
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>
-rw-r--r--mm/swap.c1
1 files changed, 0 insertions, 1 deletions
diff --git a/mm/swap.c b/mm/swap.c
index a7251a8ed532..a3a0a2f1f7c3 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,