Subject: Re: [PATCHv3 4/5] mm: make compound_head() robust
To: Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill@shutemov.name>
References: <1439976106-137226-1-git-send-email-kirill.shutemov@linux.intel.com>
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 <20150820163643.dd87de0c1a73cb63866b2914@linux-foundation.org>
 <20150821121028.GB12016@node.dhcp.inet.fi>
 <alpine.DEB.2.11.1508211109460.27769@east.gentwo.org>
 <20150821193109.GA14785@node.dhcp.inet.fi>
 <20150821123458.b3a6947135d5b506a34abc61@linux-foundation.org>
Cc: Christoph Lameter <cl@linux.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Hugh Dickins <hughd@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Johannes Weiner <hannes@cmpxchg.org>, Michal Hocko <mhocko@suse.cz>,
        David Rientjes <rientjes@google.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
From: Vlastimil Babka <vbabka@suse.cz>
Message-ID: <55DB3D19.8070009@suse.cz>
Date: Mon, 24 Aug 2015 17:49:45 +0200
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On 08/21/2015 09:34 PM, Andrew Morton wrote:
> On Fri, 21 Aug 2015 22:31:09 +0300 "Kirill A. Shutemov" <kirill@shutemov.name> wrote:
>
>> On Fri, Aug 21, 2015 at 11:11:27AM -0500, Christoph Lameter wrote:
>>> On Fri, 21 Aug 2015, Kirill A. Shutemov wrote:
>>>
>>>>> Is this really true?  For example if it's a slab page, will that page
>>>>> ever be inspected by code which is looking for the PageTail bit?
>>>>
>>>> +Christoph.
>>>>
>>>> What we know for sure is that space is not used in tail pages, otherwise
>>>> it would collide with current compound_dtor.
>>>
>>> Sl*b allocators only do a virt_to_head_page on tail pages.
>>
>> The question was whether it's safe to assume that the bit 0 is always zero
>> in the word as this bit will encode PageTail().
>
> That wasn't my question actually...
>
> What I'm wondering is: if this page is being used for slab, will any
> code path ever run PageTail() against it?  If not, we don't need to be
> concerned about that bit.

Pfn scanners such as compaction might inspect such pages and run 
compound_head() (and thus PageTail) on them. I think no kind of page 
within a zone (slab or otherwise) is "protected" from this, which is why 
it needs to be robust.

> And slab was just the example I chose.  The same question petains to
> all other uses of that union.
>

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