Re: PATCH? introduce get_compound_page (Was: process 'stuck' atexit)

[Oleg Nesterov]

On 12/16, Oleg Nesterov wrote:
>
> On 12/16, Andrea Arcangeli wrote:
> >
> > On Mon, Dec 16, 2013 at 07:36:18PM +0100, Oleg Nesterov wrote:
> > >
> > > And compound_lock_irqsave() looks racy even after get_page_unless_zero().
> > >
> > > For example, suppose that page_head was already freed and then re-allocated
> > > as (say) alloc_pages(__GFP_COMP, 1). get_page_unless_zero() can succeed right
> > > after prep_new_page() does set_page_refcounted(). Now, can't compound_lock()
> > > race with the non-atomic prep_compound_page()->__SetPageHead() ?
> >
> > Yes. We need to change to:
> >
> > 	if (order && (gfp_flags & __GFP_COMP))
> > 		prep_compound_page(page, order);
> > 	smp_wmb();
> > 	/* as the compound_lock can be taken after it's refcounted */
> > 	set_page_refcounted(page);
> >
> > __SetPageHead uses bts asm insn so literally only a "lock" prefix is
> > missing in a assembly instruction. So the race window is incredibly
> > small, but it must be fixed indeed. This also puts set_page_refcounted
> > as the last action of buffered_rmqueue so there shouldn't be any other
> > issues like this left in the page allocation code.
> >
> > Can you reorder set_page_refcount in your v2?
>
> OK. I'll try to make something on Wednesday.

Yes, I will, but...

I can't stop thinking about another change. What if we simply change
__split_huge_page_refcount() to also do compound_lock/unlock(page_tail)
in a main loop?

This way we can greatly simplify get/put_page paths, we can rely on
compound_lock(sub-page) and avoid get_page_unless_zero(page_head).
Yes, this will make _split a bit slower, but PG_compound_lock should
not be contended? And we should change page_tail->flags carefully, but
this looks simple.

Or this is not possible/desirable?

Oleg.

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