linux-kernel - Re: [PATCH RFC 00/15] decouple pagefault_disable() from preempt

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Date:	Wed, 6 May 2015 15:01:58 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Hildenbrand <dahi@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...hat.com,
	peterz@...radead.org, yang.shi@...driver.com,
	bigeasy@...utronix.de, benh@...nel.crashing.org, paulus@...ba.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 00/15] decouple pagefault_disable() from
 preempt_disable()

On Wed,  6 May 2015 19:50:24 +0200 David Hildenbrand <dahi@...ux.vnet.ibm.com> wrote:

> As Peter asked me to also do the decoupling in one shot, this is
> the new series.
> 
> I recently discovered that might_fault() doesn't call might_sleep()
> anymore. Therefore bugs like:
> 
>   spin_lock(&lock);
>   rc = copy_to_user(...);
>   spin_unlock(&lock);
> 
> would not be detected with CONFIG_DEBUG_ATOMIC_SLEEP. The code was
> changed to disable false positives for code like:
> 
>   pagefault_disable();
>   rc = copy_to_user(...);
>   pagefault_enable();
> 
> Whereby the caller wants do deal with failures.

hm, that was a significant screwup.  I wonder how many bugs we
subsequently added.

>
> ..
>

> This series therefore does 2 things:
> 
> 
> 1. Decouple pagefault_disable() from preempt_enable()
> 
> ...
> 
> 2. Reenable might_sleep() checks for might_fault()

All seems sensible to me.  pagefault_disabled has to go into the
task_struct (rather than being per-cpu) because
pagefault_disabled_inc() doesn't disable preemption, yes?

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