Date: Thu, 27 Aug 2015 20:06:07 +0200
From: Michal Hocko <mhocko@kernel.org>
To: Hugh Dickins <hughd@google.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.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: <20150827180606.GA29584@dhcp22.suse.cz>
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On Thu 27-08-15 10:28:48, Hugh Dickins wrote:
> On Thu, 27 Aug 2015, Michal Hocko wrote:
> > On Thu 27-08-15 17:09:17, Michal Hocko wrote:
> > [...]
> > > Btw. Do we need the same think for page::mapping and KSM?
> > 
> > I guess we are safe here because the address for mappings comes from
> > kmalloc and that aligned properly, right?
> 
> Not quite right, in fact.  Because usually the struct address_space
> is embedded within the struct inode (at i_data), and the struct inode
> embedded within the fs-dependent inode, and that's what's kmalloc'ed.
> 
> What makes the mapping pointer low bits safe is include/linux/fs.h:
> struct address_space {
> 	...
> } __attribute__((aligned(sizeof(long))));

Oh, right you are.

> Which we first had to add in for the cris architecture, which stumbled
> not on a genuine allocated address_space, but on that funny statically
> declared swapper_space in mm/swap_state.c.

Thanks for the clarification.

> But struct anon_vma and KSM's struct stable_node (which depend on
> the same scheme for low bits of page->mapping) have no such alignment
> attribute specified: those ones are indeed relying on the kmalloc
> guarantee as you suppose.
> 
> Does struct rcu_head have no __attribute__((aligned(whatever)))?
> Perhaps that attribute should be added when it's needed.

That's basically what I meant in the previous email.
-- 
Michal Hocko
SUSE Labs
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