linux-kernel - Re: [PATCHv3 4/5] mm: make compound

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Date:	Tue, 25 Aug 2015 22:46:44 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	paulmck@...ux.vnet.ibm.com,
	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Hugh Dickins <hughd@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	David Rientjes <rientjes@...gle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Christoph Lameter <cl@...ux.com>
Subject: Re: [PATCHv3 4/5] mm: make compound_head() robust

On 25.8.2015 22:11, Paul E. McKenney wrote:
> On Tue, Aug 25, 2015 at 09:33:54PM +0300, Kirill A. Shutemov wrote:
>> On Tue, Aug 25, 2015 at 01:44:13PM +0200, Vlastimil Babka wrote:
>>> On 08/21/2015 02:10 PM, Kirill A. Shutemov wrote:
>>>> On Thu, Aug 20, 2015 at 04:36:43PM -0700, Andrew Morton wrote:
>>>>> On Wed, 19 Aug 2015 12:21:45 +0300 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> wrote:
>>>>>
>>>>>> The patch introduces page->compound_head into third double word block in
>>>>>> front of compound_dtor and compound_order. That means it shares storage
>>>>>> space with:
>>>>>>
>>>>>>  - page->lru.next;
>>>>>>  - page->next;
>>>>>>  - page->rcu_head.next;
>>>>>>  - page->pmd_huge_pte;
>>>>>>
>>>
>>> We should probably ask Paul about the chances that rcu_head.next would like
>>> to use the bit too one day?
>>
>> +Paul.
> 
> The call_rcu() function does stomp that bit, but if you stop using that
> bit before you invoke call_rcu(), no problem.

You mean that it sets the bit 0 of rcu_head.next during its processing? That's
bad news then. It's not that we would trigger that bit when the rcu_head part of
the union is "active". It's that pfn scanners could inspect such page at
arbitrary time, see the bit 0 set (due to RCU processing) and think that it's a
tail page of a compound page, and interpret the rest of the pointer as a pointer
to the head page (to test it for flags etc).


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