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

From: Vlastimil Babka <vbabka@suse.cz>
To: "Kirill A. Shutemov" <kirill@shutemov.name>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	"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,
	Christoph Lameter <cl@linux.com>
Subject: Re: [PATCHv3 4/5] mm: make compound_head() robust
Date: Wed, 26 Aug 2015 17:39:13 +0200	[thread overview]
Message-ID: <55DDDDA1.4090407@suse.cz> (raw)
In-Reply-To: <20150826150412.GA16412@node.dhcp.inet.fi>

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...