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

[Paul E. McKenney]

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.

> 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.

							Thanx, Paul

> Btw. Do we need the same think for page::mapping and KSM?
> -- 
> Michal Hocko
> SUSE Labs
> 

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