Re: [PATCH] Remove unnecessary smp_wmb from clear_user_highpage()

[Nick Piggin <npiggin@suse.de>]

On Wed, Jul 18, 2007 at 05:45:22PM +0100, Hugh Dickins wrote:
> On Wed, 18 Jul 2007, Mel Gorman wrote:
> > 
> > At the nudging of Andrew, I was checking to see if the architecture-specific
> > implementations of alloc_zeroed_user_highpage() can be removed or not.
> 
> Ah, so that was part of the deal for getting MOVABLE in, eh ;-?
> 
> > With the exception of barriers, the differences are negligible and the main
> > memory barrier is in clear_user_highpage(). However, it's unclear why it's
> > needed. Do you mind looking at the following patch and telling me if it's
> > wrong and if so, why?
> > 
> > Thanks a lot.
> 
> I laugh when someone approaches me with a question on barriers ;)
> I usually get confused and have to go ask someone else.
> 
> And I should really to leave this query to Nick: he'll be glad of the
> opportunity to post his PageUptodate memorder patches again (looking
> in my mailbox I see versions from February, but I'm pretty sure he put
> out a more compact, less scary one later on).  He contends that the
> barrier in clear_user_highpage should not be there, but instead
> barriers (usually) needed when setting and testing PageUptodate.

And btw. (I don't think you're confused, but the last sentence could
be mislreading to readers)... I don't contend the barrier should not be
there in that it is _technically_ wrong... but logicaly the condition
we are interested in is whether the page is uptodate or not (the fact
that we only ever have uptodate pages in ptes *cough*, and the causal
dependency on *pte -> page means we don't bother setting or checking
PageUptodate for anonymous faults, but the logical condition we want
is that the page is uptodate).

So when I found that both ordering problems (fault and read(2)) could
be solved with PageUptodate, it just seems like a better place to
put it than in clear_user_highpage.

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