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

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Message-ID: <20141127120441.GB4390@osiris>
Date:	Thu, 27 Nov 2014 13:04:41 +0100
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	David Hildenbrand <dahi@...ux.vnet.ibm.com>
Cc:	"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

On Thu, Nov 27, 2014 at 09:03:01AM +0100, David Hildenbrand wrote:
> > Code like
> > 	spin_lock(&lock);
> > 	if (copy_to_user(...))
> > 		rc = ...
> > 	spin_unlock(&lock);
> > really *should* generate warnings like it did before.
> > 
> > And *only* code like
> > 	spin_lock(&lock);
> 
> Is only code like this valid or also with the spin_lock() dropped?
> (e.g. the access in patch1 if I remember correctly)
> 
> So should page_fault_disable() increment the pagefault counter and the preempt
> counter or only the first one?

Given that a sequence like

	page_fault_disable();
 	if (copy_to_user(...))
 		rc = ...
 	page_fault_enable();

is correct code right now I think page_fault_disable() should increase both.
No need for surprising semantic changes.

> So we would have pagefault code rely on:
> 
> in_disabled_pagefault() ( pagefault_disabled() ... whatever ) instead of
> in_atomic().

No, let's be more defensive: the page fault handler should do nothing if
in_atomic() just like now. But it could have a quick check and emit a one
time warning if page faults aren't disabled in addition.
That might help debugging but keeps the system more likely alive.

might_fault() however should call might_sleep() if page faults aren't
disabled, but that's what you proposed anyway I think.

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