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

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Message-ID: <20150507102254.GE23123@twins.programming.kicks-ass.net>
Date:	Thu, 7 May 2015 12:22:54 +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 Wed, May 06, 2015 at 07:50:25PM +0200, David Hildenbrand wrote:
> +/*
> + * Is the pagefault handler disabled? If so, user access methods will not sleep.
> + */
> +#define pagefault_disabled() (current->pagefault_disabled != 0)

So -RT has:

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

AFAICR we did this to avoid having to do both:

	preempt_disable();
	pagefault_disable();

in a fair number of places -- just like this patch-set does, this is
touching two cachelines where one would have been enough.

Also, removing in_atomic() from fault handlers like you did
significantly changes semantics for interrupts (soft, hard and NMI).

So while I agree with most of these patches, I'm very hesitant on the
above little detail.
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