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

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
Date: Thu, 27 Aug 2015 20:14:35 +0200	[thread overview]
Message-ID: <20150827181434.GB29584@dhcp22.suse.cz> (raw)
In-Reply-To: <20150827163634.GD4029@linux.vnet.ibm.com>

On Thu 27-08-15 09:36:34, Paul E. McKenney wrote:
> On Thu, Aug 27, 2015 at 05:09:17PM +0200, Michal Hocko wrote:
> > 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?
> 
> As long as it uses call_rcu(), call_rcu_bh(), call_rcu_sched(),
> or call_srcu() and not some future call_rcu_lazy(), no problem.
> 
> But yes, if you are going to assume that RCU leaves the bottom
> bit of the rcu_head structure's ->next field zero, then everything
> everywhere in the kernel might in the future need to be careful of
> exactly what variant of call_rcu() is used.

OK, so it would be call_rcu_$special to use the bit. This wasn't entirely
clear to me. I thought it would be opposite.

> > 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?
> 
> There are some architectures that guarantee only 16-bit alignment.
> If those architectures are fixed to do 32-bit alignment, or if support
> for them is dropped, then the future restrictions mentioned above could
> be dropped.

My understanding of the discussion which led to the above patch is that
m68k allows for 32b alignment you just have to be explicit about that
(http://thread.gmane.org/gmane.linux.ports.m68k/5932/focus=5960). Which
other archs would be affected?

I mean, this patch allows for quite some simplification in the mm code.
And I think that RCU can live with mm of the low bits without any
issues. You've said that one bit should be sufficient for the RCU use
case. So having 2 bits sounds like a good thing.
-- 
Michal Hocko
SUSE Labs