linux-kernel - Re: [PATCHv3 4/5] mm: make compound

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Message-ID: <20150826212916.GG11078@linux.vnet.ibm.com>
Date:	Wed, 26 Aug 2015 14:29:16 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Vlastimil Babka <vbabka@...e.cz>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	David Rientjes <rientjes@...gle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Christoph Lameter <cl@...ux.com>
Subject: Re: [PATCHv3 4/5] mm: make compound_head() robust

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@...ux.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.

							Thanx, Paul

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