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

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Message-ID: <49C17E22.9040807@redhat.com>
Date:	Thu, 19 Mar 2009 01:05:06 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Nick Piggin <nickpiggin@...oo.com.au>,
	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

Jeremy Fitzhardinge wrote:
> Avi Kivity wrote:
>>> Hm, awkward if flush_tlb_others doesn't IPI...
>>>
>>
>> How can it avoid flushing the tlb on cpu [01]?  It's it's 
>> gup_fast()ing a pte, it may as well load it into the tlb.
>
> xen_flush_tlb_others uses a hypercall rather than an IPI, so none of 
> the logic which depends on there being an IPI will work.

Right, of course, that's what we were talking about.  I thought 
optimizations to avoid IPIs if an mm never visited a cpu.

>
>>> Simplest fix is to make gup_get_pte() a pvop, but that does seem 
>>> like putting a red flag in front of an inner-loop hotspot, or 
>>> something...
>>>
>>> The per-cpu tlb-flush exclusion flag might really be the way to go.
>>
>> I don't see how it will work, without changing Xen to look at the flag?
>>
>> local_irq_disable() is used here to lock out a remote cpu, I don't 
>> see why deferring the flush helps.
>
> Well, no, not deferring.  Making xen_flush_tlb_others() spin waiting 
> for "doing_gup" to clear on the target cpu.  Or add an explicit notion 
> of a "pte update barrier" rather than implicitly relying on the tlb 
> IPI (which is extremely convenient when available...).

Pick up a percpu flag from all cpus and spin on each?  Nasty.

You could use the irq enabled flag; it's available and what native spins 
on (but also means I'll need to add one if I implement this).

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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