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

[Paul E. McKenney]

On Wed, Aug 26, 2015 at 03:28:39PM -0700, Hugh Dickins wrote:
> 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@xxxxxxxxxxxxxxx> 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.

Whew!  ;-)

							Thanx, Paul

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