linux-kernel - Re: [PATCH] mm: disable preemption in apply_to_pte

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Message-ID: <4995ACD5.9000201@goop.org>
Date:	Fri, 13 Feb 2009 09:24:37 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Nick Piggin <nickpiggin@...oo.com.au>, linux-mm@...ck.org,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] mm: disable preemption in apply_to_pte_range

Peter Zijlstra wrote:
>> The specific rules are that 
>> arch_enter_lazy_mmu_mode()/arch_leave_lazy_mmu_mode() require you to be 
>> holding the appropriate pte locks for the ptes you're updating, so 
>> preemption is naturally disabled in that case.
>>     
>
> Right, except on -rt where the pte lock is a mutex.
>   

Hm, that's interesting.  The requirement isn't really "no preemption", 
its "must not migrate to another cpu".  Is there a better way to express 
that?

>> This all goes a bit strange with init_mm's non-requirement for taking 
>> pte locks.  The caller has to arrange for some kind of serialization on 
>> updating the range in question, and that could be a mutex.  Explicitly 
>> disabling preemption in enter_lazy_mmu_mode would make sense for this 
>> case, but it would be redundant for the common case of batched updates 
>> to usermode ptes.
>>     
>
> I really utterly hate how you just plonk preempt_disable() in there
> unconditionally and without very clear comments on how and why.
>   

Well, there's the commit comment.  They're important, right?  That's why 
we spend time writing good commit comments?  So they get read?  ;)

OK, I'll add a comment, particularly if there's a more precise way to 
express "no migration".

> I'd rather we'd fix up the init_mm to also have a pte lock.
>   
Yes, I don't like the init_mm-exceptionalism either.

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