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SubjectRe: [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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