linux-kernel - Re: [RFC][PATCH 5/6] mm: Provide speculative fault infrastructure

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Date:	Tue, 21 Oct 2014 21:00:19 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	torvalds@...ux-foundation.org, paulmck@...ux.vnet.ibm.com,
	tglx@...utronix.de, akpm@...ux-foundation.org, riel@...hat.com,
	mgorman@...e.de, oleg@...hat.com, mingo@...hat.com,
	minchan@...nel.org, kamezawa.hiroyu@...fujitsu.com,
	viro@...iv.linux.org.uk, laijs@...fujitsu.com, dave@...olabs.net
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC][PATCH 5/6] mm: Provide speculative fault infrastructure

On Mon, Oct 20, 2014 at 11:56:38PM +0200, Peter Zijlstra wrote:
>  static bool pte_map_lock(struct fault_env *fe)
>  {
> +	bool ret = false;
> +
> +	if (!(fe->flags & FAULT_FLAG_SPECULATIVE)) {
> +		fe->pte = pte_offset_map_lock(fe->mm, fe->pmd, fe->address, &fe->ptl);
> +		return true;
> +	}
> +
> +	/*
> +	 * The first vma_is_dead() guarantees the page-tables are still valid,
> +	 * having IRQs disabled ensures they stay around, hence the second
> +	 * vma_is_dead() to make sure they are still valid once we've got the
> +	 * lock. After that a concurrent zap_pte_range() will block on the PTL
> +	 * and thus we're safe.
> +	 */
> +	local_irq_disable();
> +	if (vma_is_dead(fe->vma, fe->sequence))
> +		goto out;
> +
>  	fe->pte = pte_offset_map_lock(fe->mm, fe->pmd, fe->address, &fe->ptl);

Yeah, so this deadlocks just fine, I found we still do TLB flushes while
holding the PTL. Bugger that, the alternative is either force everybody
to do RCU freed page-tables or put back the ugly code :/

A well..

> +
> +	if (vma_is_dead(fe->vma, fe->sequence)) {
> +		pte_unmap_unlock(fe->pte, fe->ptl);
> +		goto out;
> +	}
> +
> +	ret = true;
> +out:
> +	local_irq_enable();
> +	return ret;
>  }
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