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

From: Hugh Dickins <hughd@google.com>
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>,
	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 15:28:39 -0700 (PDT)	[thread overview]
Message-ID: <alpine.LSU.2.11.1508261526260.2585@eggly.anvils> (raw)
In-Reply-To: <20150826212916.GG11078@linux.vnet.ibm.com>

On Wed, 26 Aug 2015, Paul E. McKenney wrote:
> On Wed, Aug 26, 2015 at 11:18:45AM -0700, Hugh Dickins wrote:
> > On Tue, 25 Aug 2015, Paul E. McKenney wrote:
> > > On Tue, Aug 25, 2015 at 10:46:44PM +0200, Vlastimil Babka wrote:
> > > > On 25.8.2015 22:11, Paul E. McKenney wrote:
> > > > > On Tue, Aug 25, 2015 at 09:33:54PM +0300, Kirill A. Shutemov wrote:
> > > > >> On Tue, Aug 25, 2015 at 01:44:13PM +0200, Vlastimil Babka wrote:
> > > > >>> On 08/21/2015 02:10 PM, Kirill A. Shutemov wrote:
> > > > >>>> On Thu, Aug 20, 2015 at 04:36:43PM -0700, Andrew Morton wrote:
> > > > >>>>> On Wed, 19 Aug 2015 12:21:45 +0300 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:
> > > > >>>>>
> > > > >>>>>> The patch introduces page->compound_head into third double word block in
> > > > >>>>>> front of compound_dtor and compound_order. That means it shares storage
> > > > >>>>>> space with:
> > > > >>>>>>
> > > > >>>>>>  - page->lru.next;
> > > > >>>>>>  - page->next;
> > > > >>>>>>  - page->rcu_head.next;
> > > > >>>>>>  - page->pmd_huge_pte;
> > > > >>>>>>
> > > > >>>
> > > > >>> We should probably ask Paul about the chances that rcu_head.next would like
> > > > >>> to use the bit too one day?
> > > > >>
> > > > >> +Paul.
> > > > > 
> > > > > The call_rcu() function does stomp that bit, but if you stop using that
> > > > > bit before you invoke call_rcu(), no problem.
> > > > 
> > > > You mean that it sets the bit 0 of rcu_head.next during its processing?
> > > 
> > > Not at the moment, though RCU will splat if given a misaligned rcu_head
> > > structure because of the possibility to use that bit to flag callbacks
> > > that do nothing but free memory.  If RCU needs to do that (e.g., to
> > > promote energy efficiency), then that bit might well be set during
> > > RCU grace-period processing.
> > 
> > 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.

That's exactly what we want: sounds like we have no problem, thanks Paul.

Hugh