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

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Date:	Wed, 26 Aug 2015 17:39:13 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	"Kirill A. Shutemov" <kirill@...temov.name>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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 08/26/2015 05:04 PM, Kirill A. Shutemov wrote:
>>>                                                                          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).
>>
>> On the other hand, if you avoid scanning rcu_head structures for pages
>> that are currently waiting for a grace period, no problem.  RCU does
>> not use the rcu_head structure at all except for during the time between
>> when call_rcu() is invoked on that rcu_head structure and the time that
>> the callback is invoked.
>>
>> Is there some other page state that indicates that the page is waiting
>> for a grace period?  If so, you could simply avoid testing that bit in
>> that case.
>
> No, I don't think so.
>
> For compound pages most of info of its state is stored in head page (e.g.
> page_count(), flags, etc). So if we examine random page (pfn scanner case)
> the very first thing we want to know if we stepped on tail page.
> PageTail() is what I wanted to encode in the bit...
>
> What if we change order of fields within rcu_head and put ->func first?

Or change the order of compound_head wrt the rest?

> Can we expect this pointer to have bit 0 always clear?

That's probably a question whether $compiler is guaranteed to align 
functions on all architectures...
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