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

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Message-ID: <20150507082346.3e8e045e@thinkpad-w530>
Date:	Thu, 7 May 2015 08:23:46 +0200
From:	David Hildenbrand <dahi@...ux.vnet.ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
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()

> > 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?
> 

Right, we can now get scheduled while in pagefault_disable() (if preemption
hasn't been disabled manually). So we have to store it per task/thread not per
cpu.

Actually even the preempt disable counter is only per-cpu for x86 and lives in
thread_info for all other archs (which is also not 100% clean but doesn't
matter at that point).

I had that pagefault disable counter in thread_info before, but that required
messing with asm-offsets of some arch (I had a proper version but this one
feels cleaner).

Thanks for having a look!

David

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