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

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Message-ID: <20150507134030.137deeb2@thinkpad-w530>
Date:	Thu, 7 May 2015 13:40:30 +0200
From:	David Hildenbrand <dahi@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 12:50:53PM +0200, David Hildenbrand wrote:
> > Just to make sure we have a common understanding (as written in my cover
> > letter):
> > 
> > Your suggestion won't work with !CONFIG_PREEMPT (!CONFIG_PREEMPT_COUNT). If
> > there is no preempt counter, in_atomic() won't work. 
> 
> But there is, we _always_ have a preempt_count, and irq_enter() et al.
> _always_ increment the relevant bits.
> 
> The thread_info::preempt_count field it never under PREEMPT_COUNT
> include/asm-generic/preempt.h provides stuff regardless of
> PREEMPT_COUNT.
> 
> See how __irq_enter() -> preempt_count_add(HARDIRQ_OFFSET) ->
> __preempt_count_add() _always_ just works.


Okay thinking about this further, I think I got your point. That basically means
that the in_atomic() check makes sense for irqs.

But in my opinion, it does not help do replace

preempt_disable()
pagefault_disable()

by

preempt_disable()


(as discussed because of the PREEMPT_COUNT stuff)

So I agree that we should better add it to not mess with hard/soft irq.

> 
> Its only things like preempt_disable() / preempt_enable() that get
> munged depending on PREEMPT_COUNT/PREEMPT.
> 

But anyhow, opinions seem to differ how to best handle that whole stuff.

I think a separate counter just makes sense, as we are dealing with two
different concepts and we don't want to lose the preempt_disable =^ NOP
for !CONFIG_PREEMPT.

I also think that

pagefault_disable()
rt = copy_from_user()
pagefault_enable()

is a valid use case.

So any suggestions how to continue?

Thanks!

David

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