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

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Date:	Mon, 23 Dec 2013 12:43:00 +0100
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	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>,
	Mel Gorman <mgorman@...e.de>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH 1/1] mm: fix the theoretical compound_lock() vs
 prep_new_page() race

On Thu, Dec 19, 2013 at 08:09:20PM +0100, Oleg Nesterov wrote:
> get/put_page(thp_tail) paths do get_page_unless_zero(page_head) +
> compound_lock(). In theory this page_head can be already freed and
> reallocated as alloc_pages(__GFP_COMP, smaller_order). In this case
> get_page_unless_zero() can succeed right after set_page_refcounted(),
> and compound_lock() can race with the non-atomic __SetPageHead().
> 
> Perhaps we should rework the thp locking (under discussion), but
> until then this patch moves set_page_refcounted() and adds wmb()
> to ensure that page->_count != 0 comes as a last change.
> 
> I am not sure about other callers of set_page_refcounted(), but at
> first glance they look fine to me.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Acked-by: Andrea Arcangeli <aarcange@...hat.com>

Only one improvement possible, the smp_wmb() could have been put under
CONFIG_TRANSPARENT_HUGEPAGE somehow. No difference for x86-64 though.

Thanks,
Andrea
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