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

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Message-Id: <1234525710.6519.17.camel@twins>
Date:	Fri, 13 Feb 2009 12:48:30 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jeremy Fitzhardinge <jeremy@...p.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

On Thu, 2009-02-12 at 17:39 -0800, Jeremy Fitzhardinge wrote:

> In general the model for lazy updates is that you're batching the 
> updates in some queue somewhere, which is almost certainly a piece of 
> percpu state being maintained by someone.  Its therefore broken and/or 
> meaningless to have the code making the updates wandering between cpus 
> for the duration of the lazy updates.
> 
> > If so, should we do the preempt_disable/enable within those functions? 
> > Probably not worth the cost, I guess.
> 
> 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.

> 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.

I'd rather we'd fix up the init_mm to also have a pte lock.
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