linux-kernel - Re: [PATCH v6 08/12] Handle async PF in a guest.

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Message-ID: <4CADC6C3.3040305@redhat.com>
Date:	Thu, 07 Oct 2010 15:10:27 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	kvm@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, mingo@...e.hu,
	a.p.zijlstra@...llo.nl, tglx@...utronix.de, hpa@...or.com,
	riel@...hat.com, cl@...ux-foundation.org, mtosatti@...hat.com
Subject: Re: [PATCH v6 08/12] Handle async PF in a guest.

  On 10/04/2010 05:56 PM, Gleb Natapov wrote:
> When async PF capability is detected hook up special page fault handler
> that will handle async page fault events and bypass other page faults to
> regular page fault handler. Also add async PF handling to nested SVM
> emulation. Async PF always generates exit to L1 where vcpu thread will
> be scheduled out until page is available.
>

Please separate guest and host changes.

> +void kvm_async_pf_task_wait(u32 token)
> +{
> +	u32 key = hash_32(token, KVM_TASK_SLEEP_HASHBITS);
> +	struct kvm_task_sleep_head *b =&async_pf_sleepers[key];
> +	struct kvm_task_sleep_node n, *e;
> +	DEFINE_WAIT(wait);
> +
> +	spin_lock(&b->lock);
> +	e = _find_apf_task(b, token);
> +	if (e) {
> +		/* dummy entry exist ->  wake up was delivered ahead of PF */
> +		hlist_del(&e->link);
> +		kfree(e);
> +		spin_unlock(&b->lock);
> +		return;
> +	}
> +
> +	n.token = token;
> +	n.cpu = smp_processor_id();
> +	init_waitqueue_head(&n.wq);
> +	hlist_add_head(&n.link,&b->list);
> +	spin_unlock(&b->lock);
> +
> +	for (;;) {
> +		prepare_to_wait(&n.wq,&wait, TASK_UNINTERRUPTIBLE);
> +		if (hlist_unhashed(&n.link))
> +			break;
> +		local_irq_enable();

Suppose we take another apf here.  And another, and another (for 
different pages, while executing schedule()).  What's to prevent kernel 
stack overflow?

> +		schedule();
> +		local_irq_disable();
> +	}
> +	finish_wait(&n.wq,&wait);
> +
> +	return;
> +}
> +EXPORT_SYMBOL_GPL(kvm_async_pf_task_wait);
> +
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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