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

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Message-ID: <20141127164555.4bcebfe8@thinkpad-w530>
Date:	Thu, 27 Nov 2014 16:45:55 +0100
From:	David Hildenbrand <dahi@...ux.vnet.ibm.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Christian Borntraeger" <borntraeger@...ibm.com>,
	"paulus@...ba.org" <paulus@...ba.org>,
	"schwidefsky@...ibm.com" <schwidefsky@...ibm.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"mingo@...nel.org" <mingo@...nel.org>
Subject: Re: [RFC 0/2] Reenable might_sleep() checks for might_fault() when
 atomic

> From: David Hildenbrand
> ...
> > 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.
> 
> What about adding (say) 0x10000 for the more restrictive test?
> 
> 	David
> 

You mean as part of the preempt counter?

The current layout (on my branch) is

 * PREEMPT_MASK:        0x000000ff
 * SOFTIRQ_MASK:        0x0000ff00
 * HARDIRQ_MASK:        0x000f0000
 *     NMI_MASK:        0x00100000
 * PREEMPT_ACTIVE:      0x00200000

I would have added
 * PAGEFAULT_MASK:      0x03C00000

So 4 bit == 16 levels (tbd)

By implementing scope checks in the debug case like done for the regular
preempt_count_inc() preempt_count_dec(), we could catch over/underflows.

Thanks,

David

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