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

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Message-ID: <20150507115118.GT21418@twins.programming.kicks-ass.net>
Date:	Thu, 7 May 2015 13:51:18 +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 01:40:30PM +0200, David Hildenbrand wrote:
> 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?


static inline bool __pagefault_disabled(void)
{
	return current->pagefault_disabled;
}

static inline bool pagefault_disabled(void)
{
	return in_atomic() || __pagefault_disabled();
}

And leave the preempt_disable() + pagefault_disable() for now. You're
right in that that is clearest.

If we ever get to the point where that really is an issue, I'll try and
be clever then :-)
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