Date: Thu, 27 Aug 2015 17:09:17 +0200
From: Michal Hocko <mhocko@kernel.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Hugh Dickins <hughd@google.com>, Vlastimil Babka <vbabka@suse.cz>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        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
Message-ID: <20150827150917.GF27052@dhcp22.suse.cz>
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On Wed 26-08-15 14:29:16, Paul E. McKenney wrote:
> On Wed, Aug 26, 2015 at 11:18:45AM -0700, Hugh Dickins wrote:
[...]
> > But if you do one day implement that, wouldn't sl?b.c have to use
> > call_rcu_with_added_meaning() instead of call_rcu(), to be in danger
> > of getting that bit set?  (No rcu_head is placed in a PageTail page.)
> 
> Good point, call_rcu_lazy(), but yes.
> 
> > So although it might be a little strange not to use a variant intended
> > for freeing memory when indeed that's what it's doing, it would not be
> > the end of the world for SLAB_DESTROY_BY_RCU to carry on using straight
> > call_rcu(), in defence of the struct page safety Kirill is proposing.
> 
> As long as you are OK with the bottom bit being zero throughout the RCU
> processing, yes.

I am really not sure I udnerstand. What will prevent
call_rcu(&page->rcu_head, free_page_rcu) done in a random driver?

Cannot the RCU simply claim bit1? I can see 1146edcbef37 ("rcu: Loosen
__call_rcu()'s rcu_head alignment constraint") but AFAIU all it would
take to fix this would be to require struct rcu_head to be aligned to
32b no?

Btw. Do we need the same think for page::mapping and KSM?
-- 
Michal Hocko
SUSE Labs
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