linux-kernel - Re: [RFC 0/2] Reenable might_sleep() checks for might

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Date:	Thu, 27 Nov 2014 16:19:05 +0100
From:	David Hildenbrand <dahi@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	linuxppc-dev@...ts.ozlabs.org, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, benh@...nel.crashing.org,
	paulus@...ba.org, akpm@...ux-foundation.org,
	schwidefsky@...ibm.com, mingo@...nel.org
Subject: Re: [RFC 0/2] Reenable might_sleep() checks for might_fault() when
 atomic

> OTOH, there is no reason why we need to disable preemption over that
> page_fault_disabled() region. There are code pathes which really do
> not require to disable preemption for that.
> 
> We have that seperated in preempt-rt for obvious reasons and IIRC
> Peter Zijlstra tried to distangle it in mainline some time ago. I
> forgot why that never got merged.
> 

Of course, we can completely separate that in our page fault code by doing
pagefault_disabled() checks instead of in_atomic() checks (even in add on
patches later).

> We tie way too much stuff on the preemption count already, which is a
> mightmare because we have no clear distinction of protection
> scopes. 

Although it might not be optimal, but keeping a separate counter for
pagefault_disable() as part of the preemption counter seems to be the only
doable thing right now. I am not sure if a completely separated counter is even
possible, increasing the size of thread_info.

I am working on a prototype right now.

Thanks!

> 
> Thanks,
> 
> 	tglx
> 

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