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Re: [PATCHv3 4/5] mm: make compound_head() robust

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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@linux.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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Re: [PATCHv3 4/5] mm: make compound_head() robust Andrew Morton <akpm@linux-foundation.org> - 2015-08-21 01:40 +0200
  Re: [PATCHv3 4/5] mm: make compound_head() robust "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-08-21 14:20 +0200
    Re: [PATCHv3 4/5] mm: make compound_head() robust Christoph Lameter <cl@linux.com> - 2015-08-21 18:20 +0200
      Re: [PATCHv3 4/5] mm: make compound_head() robust "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-08-21 21:40 +0200
        Re: [PATCHv3 4/5] mm: make compound_head() robust Andrew Morton <akpm@linux-foundation.org> - 2015-08-21 21:40 +0200
          Re: [PATCHv3 4/5] mm: make compound_head() robust Christoph Lameter <cl@linux.com> - 2015-08-21 23:20 +0200
          Re: [PATCHv3 4/5] mm: make compound_head() robust Vlastimil Babka <vbabka@suse.cz> - 2015-08-24 17:50 +0200
    Re: [PATCHv3 4/5] mm: make compound_head() robust Vlastimil Babka <vbabka@suse.cz> - 2015-08-25 13:50 +0200
      Re: [PATCHv3 4/5] mm: make compound_head() robust "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-08-25 20:40 +0200
        Re: [PATCHv3 4/5] mm: make compound_head() robust "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-08-25 22:20 +0200
          Re: [PATCHv3 4/5] mm: make compound_head() robust Vlastimil Babka <vbabka@suse.cz> - 2015-08-25 22:50 +0200
            Re: [PATCHv3 4/5] mm: make compound_head() robust "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-08-25 23:30 +0200
              Re: [PATCHv3 4/5] mm: make compound_head() robust "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-08-26 17:10 +0200
                Re: [PATCHv3 4/5] mm: make compound_head() robust Vlastimil Babka <vbabka@suse.cz> - 2015-08-26 17:40 +0200
                Re: [PATCHv3 4/5] mm: make compound_head() robust "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-08-26 18:40 +0200
              Re: [PATCHv3 4/5] mm: make compound_head() robust Hugh Dickins <hughd@google.com> - 2015-08-26 20:20 +0200
                Re: [PATCHv3 4/5] mm: make compound_head() robust "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-08-26 23:30 +0200
                Re: [PATCHv3 4/5] mm: make compound_head() robust Hugh Dickins <hughd@google.com> - 2015-08-27 00:30 +0200
                Re: [PATCHv3 4/5] mm: make compound_head() robust "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-08-27 01:40 +0200
                Re: [PATCHv3 4/5] mm: make compound_head() robust Michal Hocko <mhocko@kernel.org> - 2015-08-27 17:10 +0200
                Re: [PATCHv3 4/5] mm: make compound_head() robust Michal Hocko <mhocko@kernel.org> - 2015-08-27 18:10 +0200
                Re: [PATCHv3 4/5] mm: make compound_head() robust Hugh Dickins <hughd@google.com> - 2015-08-27 19:30 +0200
                Re: [PATCHv3 4/5] mm: make compound_head() robust Michal Hocko <mhocko@kernel.org> - 2015-08-27 20:10 +0200
                Re: [PATCHv3 4/5] mm: make compound_head() robust "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-08-27 18:40 +0200
                Re: [PATCHv3 4/5] mm: make compound_head() robust Michal Hocko <mhocko@kernel.org> - 2015-08-27 20:20 +0200
                Re: [PATCHv3 4/5] mm: make compound_head() robust "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-08-27 21:20 +0200

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