linux-kernel - Re: Question about x86/mm/gup.c's use of disabled interrupts

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Date:	Thu, 19 Mar 2009 21:40:29 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>, Avi Kivity <avi@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Jan Beulich <jbeulich@...ell.com>, Ingo Molnar <mingo@...e.hu>
Subject: Re: Question about x86/mm/gup.c's use of disabled interrupts

On Thu, Mar 19, 2009 at 10:31:55AM -0700, Jeremy Fitzhardinge wrote:
> Nick Piggin wrote:
>>> Also, assuming that disabling the interrupt is enough to get the
>>> guarantees we need here, there's a Xen problem because we don't use IPIs
>>> for cross-cpu tlb flushes (well, it happens within Xen).  I'll have to
>>> think a bit about how to deal with that, but I'm thinking that we could
>>> add a per-cpu "tlb flushes blocked" flag, and maintain some kind of
>>> per-cpu deferred tlb flush count so we can get around to doing the flush
>>> eventually.
>>>
>>> But I want to make sure I understand the exact algorithm here.
>>
>> FWIW, powerpc actually can flush tlbs without IPIs, and it also has
>> a gup_fast. powerpc RCU frees its page _tables_ so we can walk them,
>> and then I use speculative page references in order to be able to
>> take a reference on the page without having it pinned.
>
> Ah, interesting.  So disabling interrupts prevents the RCU free from 
> happening, and non-atomic pte fetching is a non-issue.  So it doesn't 
> address the PAE side of the problem.

This would be rcu_sched, correct?

							Thanx, Paul

>> Turning gup_get_pte into a pvop would be a bit nasty because on !PAE
>> it is just a single load, and even on PAE it is pretty cheap.
>>   
>
> Well, it wouldn't be too bad; for !PAE it would turn into something we 
> could inline, so there'd be little to no cost.  For PAE it would be out of 
> line, but a direct function call, which would be nicely cached and very 
> predictable once we've gone through the the loop once (and for Xen I think 
> I'd just make it a cmpxchg8b-based implementation, assuming that the tlb 
> flush hypercall would offset the cost of making gup_fast a bit slower).
>
> But it would be better if we can address it at a higher level.
>
>    J
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