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

From: Michal Hocko
Date: Thu Aug 27 2015 - 11:09:24 EST

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?

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?

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