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

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Date:	Wed, 18 Mar 2009 15:14:08 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Avi Kivity <avi@...hat.com>
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

Avi Kivity wrote:
> Jeremy Fitzhardinge wrote:
>>>> Disabling the interrupt will prevent the tlb flush IPI from coming 
>>>> in and flushing this cpu's tlb, but I don't see how it will prevent 
>>>> some other cpu from actually updating the pte in the pagetable, 
>>>> which is what we're concerned about here.  
>>>
>>> The thread that cleared the pte holds the pte lock and is now 
>>> waiting for the IPI.  The thread that wants to update the pte will 
>>> wait for the pte lock, thus also waits on the IPI and gup_fast()'s 
>>> local_irq_enable().  I think.
>>
>> But hasn't it already done the pte update at that point?
>>
>> (I think this conversation really is moot because the kernel never 
>> does P->P pte updates any more; its always P->N->P.)
>
> I thought you were concerned about cpu 0 doing a gup_fast(), cpu 1 
> doing P->N, and cpu 2 doing N->P.  In this case cpu 2 is waiting on 
> the pte lock.

The issue is that if cpu 0 is doing a gup_fast() and other cpus are 
doing P->P updates, then gup_fast() can potentially get a mix of old and 
new pte values - where P->P is any aggregate set of unsynchronized P->N 
and N->P operations on any number of other cpus.  Ah, but if every P->N 
is followed by a tlb flush, then disabling interrupts will hold off any 
following N->P, allowing gup_fast to get a consistent pte snapshot.

Hm, awkward if flush_tlb_others doesn't IPI...

> Won't stop munmap().

And I guess it does the tlb flush before freeing the pages, so disabling 
the interrupt helps here too.

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.

    J
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