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

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Date:	Wed, 26 Aug 2015 11:18:45 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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>,
	Hugh Dickins <hughd@...gle.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 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.)

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.

hUgh
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