linux-kernel - Re: [PATCH RFC 01/15] uaccess: count pagefault_disable() levels in pagefault

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Message-ID: <20150507123208.GJ23123@twins.programming.kicks-ass.net>
Date:	Thu, 7 May 2015 14:32:08 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	David Hildenbrand <dahi@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...hat.com,
	yang.shi@...driver.com, bigeasy@...utronix.de,
	benh@...nel.crashing.org, paulus@...ba.org,
	akpm@...ux-foundation.org, heiko.carstens@...ibm.com,
	schwidefsky@...ibm.com, borntraeger@...ibm.com, mst@...hat.com,
	tglx@...utronix.de, David.Laight@...LAB.COM, hughd@...gle.com,
	hocko@...e.cz, ralf@...ux-mips.org, herbert@...dor.apana.org.au,
	linux@....linux.org.uk, airlied@...ux.ie, daniel.vetter@...el.com,
	linux-mm@...ck.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH RFC 01/15] uaccess: count pagefault_disable() levels in
 pagefault_disabled

On Thu, May 07, 2015 at 02:14:39PM +0200, David Hildenbrand wrote:
> Thanks :), well just to make sure I got your opinion on this correctly:
> 
> 1. You think that 2 counters is the way to go for now

ack

> 2. You agree that we can't replace preempt_disable()+pagefault_disable() with
> preempt_disable() (CONFIG_PREEMPT stuff), so we need to have them separately

ack

> 3. We need in_atomic() (in the fault handlers only!) in addition to make sure we
> don't mess with irq contexts (In that case I would add a good comment to that
> place, describing why preempt_disable() won't help)

ack

> I think this is the right way to go because:
> 
> a) This way we don't have to modify preempt_disable() logic (including
> PREEMPT_COUNT).
> 
> b) There are not that many users relying on
> preempt_disable()+pagefault_disable()  (compared to pure preempt_disable() or
> pagefault_disable() users), so the performance overhead of two cache lines
> should be small. Users only making use of one of them should see no difference
> in performance.

indeed.

> c) We correctly decouple preemption and pagefault logic. Therefore we can now
> preempt when pagefaults are disabled, which feels right.

Right, that's always been the intent of introducing pagefault_disable().
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