The patch titled
Subject: mm: drop bogus VM_BUG_ON_PAGE assert in put_page() codepath
has been removed from the -mm tree. Its filename was
mm-drop-bogus-vm_bug_on_page-assert-in-put_page-codepath.patch
This patch was dropped because it was merged into mainline or a subsystem tree
------------------------------------------------------
From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Subject: mm: drop bogus VM_BUG_ON_PAGE assert in put_page() codepath
My patch 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@xxxxxxxxxxxxxxx>
Reported-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Reviewed-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Reported-by: Borislav Petkov <bp@xxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/swap.c | 1 -
1 file changed, 1 deletion(-)
diff -puN mm/swap.c~mm-drop-bogus-vm_bug_on_page-assert-in-put_page-codepath mm/swap.c
--- a/mm/swap.c~mm-drop-bogus-vm_bug_on_page-assert-in-put_page-codepath
+++ a/mm/swap.c
@@ -131,7 +131,6 @@ void put_unrefcounted_compound_page(stru
* 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,
_
Patches currently in -mm which might be from kirill.shutemov@xxxxxxxxxxxxxxx are
origin.patch
page-flags-trivial-cleanup-for-pagetrans-helpers.patch
page-flags-introduce-page-flags-policies-wrt-compound-pages.patch
page-flags-define-pg_locked-behavior-on-compound-pages.patch
page-flags-define-behavior-of-fs-io-related-flags-on-compound-pages.patch
page-flags-define-behavior-of-lru-related-flags-on-compound-pages.patch
page-flags-define-behavior-of-lru-related-flags-on-compound-pages-fix.patch
page-flags-define-behavior-slb-related-flags-on-compound-pages.patch
page-flags-define-behavior-of-xen-related-flags-on-compound-pages.patch
page-flags-define-pg_reserved-behavior-on-compound-pages.patch
page-flags-define-pg_swapbacked-behavior-on-compound-pages.patch
page-flags-define-pg_swapcache-behavior-on-compound-pages.patch
page-flags-define-pg_mlocked-behavior-on-compound-pages.patch
page-flags-define-pg_uncached-behavior-on-compound-pages.patch
page-flags-define-pg_uptodate-behavior-on-compound-pages.patch
page-flags-look-on-head-page-if-the-flag-is-encoded-in-page-mapping.patch
mm-sanitize-page-mapping-for-tail-pages.patch
include-linux-page-flagsh-rename-macros-to-avoid-collisions.patch
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