From 73933b3315e65d92e0c6cc4f8b4c51c9dc87546b Mon Sep 17 00:00:00 2001 From: "Kirill A. Shutemov" 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 Reported-by: Andrea Arcangeli Reviewed-by: Andrea Arcangeli Reported-by: Borislav Petkov Cc: Hugh Dickins Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/swap.c | 1 - 1 file changed, 1 deletion(-) 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,