linux-kernel - Re: [PATCH v2 1/1] mm: fix the theoretical compound_lock() vs prep_new

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Date:	Fri, 10 Jan 2014 16:12:47 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH v2 1/1] mm: fix the theoretical compound_lock() vs
 prep_new_page() race

On Thu, Jan 09, 2014 at 03:04:47PM +0100, Oleg Nesterov wrote:
> 
> > #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >         page_head = page;
> >         if (unlikely(PageTail(page))) {
> >                 put_page(page);
> >
> >
> > so I'm still not seeing how a tail page racing with a split ends up with
> > mayhem.
> 
> But get/put(page_tail) plays with page_head which can be freed/reallocated,
> it does compound_lock(page_head).
> 
> > I could also still be stuck in a "la la la, everything is fine" mode.
> 
> More likely it is me who tries to deny the fact I missed something ;)
> 

My hangup was that this was related to futex and I was focusing it as
a specific example that made the patch necessary. However, this is a
therotical case that potentially impacts a put_page if it mistakenly
believes it is still a tail page when it's not due a a parallel split. I
see and understand that race and while I think the patch is overkill, I
have no problem with including it at the start of a series that reexamines
the locking in that area. It makes for a suitable -stable backport and
I hope/expect the reworked locking would then remove the barrier again
for upstream.

I haven't looked at the reworked locking but understand there is a v3 on
the way so I'll wait until that happens and work my way through it.

Thanks and sorry for the noise.

-- 
Mel Gorman
SUSE Labs
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