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Date:	Fri, 06 Jun 2014 15:03:43 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: 3.15-rc8 oops in copy_page_rep after page fault.

On 06/06/2014 02:49 PM, Kirill A. Shutemov wrote:
> On Fri, Jun 06, 2014 at 11:26:14AM -0700, Linus Torvalds wrote:
>> > On Fri, Jun 6, 2014 at 10:43 AM, Dave Jones <davej@...hat.com> wrote:
>>> > >
>>> > > RIP: 0010:[<ffffffff8b3287b5>]  [<ffffffff8b3287b5>] copy_page_rep+0x5/0x10
>> > 
>> > Ok, it's the first iteration of "rep movsq" (%rcx is still 0x200) for
>> > copying a page, and the pages are
>> > 
>> >   RSI: ffff880052766000
>> >   RDI: ffff880014efe000
>> > 
>> > which both look like reasonable kernel addresses. So I'm assuming it's
>> > DEBUG_PAGEALLOC that makes this trigger, and since the error code is
>> > 0, and the CR2 value matches RSI, it's the source page that seems to
>> > have been freed.
>> > 
>> > And I see absolutely _zero_ reason for wht your 64k mmap_min_addr
>> > should make any difference what-so-ever. That's just odd.
>> > 
>> > Anyway, can you try to figure out _which_ copy_user_highpage() it is
>> > (by looking at what is around the call-site at
>> > "handle_mm_fault+0x1e0". The fact that we have a stale
>> > do_huge_pmd_wp_page() on the stack makes me suspect that we have hit
>> > that VM_FAULT_FALLBACK case and this is related to splitting. Adding a
>> > few more people explicitly to the cc in case anybody sees anything
>> > (original email on lkml and linux-mm for context, guys).
> Looks like a known false positive from DEBUG_PAGEALLOC:
> 
> https://lkml.org/lkml/2013/3/29/103
> 
> We huge copy page in do_huge_pmd_wp_page() without ptl taken and the page
> can be splitted and freed under us. Once page is copied we take ptl again
> and recheck that PMD is not changed. If changed, we don't use new page.
> Not a bug, never triggered with DEBUG_PAGEALLOC disabled.
> 
> It would be nice to have a way to mark this kind of speculative access.

FWIW, this issue makes fuzzing with DEBUG_PAGEALLOC nearly impossible since
this thing is so common we never get to do anything "fun" before this issue
triggers.

A fix would be more than welcome.


Thanks,
Sasha
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