Date: Thu, 27 Nov 2014 09:03:01 +0100
From: David Hildenbrand <dahi@linux.vnet.ibm.com>
To: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        linuxppc-dev@lists.ozlabs.org, linux-arch@vger.kernel.org,
        linux-kernel@vger.kernel.org, benh@kernel.crashing.org,
        paulus@samba.org, akpm@linux-foundation.org, schwidefsky@de.ibm.com,
        mingo@kernel.org
Subject: Re: [RFC 0/2] Reenable might_sleep() checks for might_fault() when
 atomic
Message-ID: <20141127090301.3ddc3077@thinkpad-w530>
In-Reply-To: <20141127070919.GA4390@osiris>
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	<5475FAB1.1000802@de.ibm.com>
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> 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?

> 	page_fault_disable();
> 	if (copy_to_user(...))
> 		rc = ...
> 	page_fault_enable();
> 	spin_unlock(&lock);
> should not generate warnings, since the author hopefully knew what he did.
> 
> We could achieve that by e.g. adding a couple of pagefault disabled bits
> within current_thread_info()->preempt_count, which would allow
> pagefault_disable() and pagefault_enable() to modify a different part of
> preempt_count than it does now, so there is a way to tell if pagefaults have
> been explicitly disabled or are just a side effect of preemption being
> disabled.
> This would allow might_fault() to restore its old sane behaviour for the
> !page_fault_disabled() case.

So we would have pagefault code rely on:

in_disabled_pagefault() ( pagefault_disabled() ... whatever ) instead of
in_atomic().

I agree with this approach, as this is basically what I suggested in one of my
previous mails.

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