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

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Date:	Thu, 19 Mar 2009 10:31:55 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
CC:	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

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.

> 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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