Re: [PATCH 03/14] KVM: arm64: Continue stage-2 map when re-creating mappings

[Marc Zyngier]

On Mon, 19 Jul 2021 11:47:24 +0100,
Quentin Perret <qperret@xxxxxxxxxx> wrote:
> 
> The stage-2 map walkers currently return -EAGAIN when re-creating
> identical mappings or only changing access permissions. This allows to
> optimize mapping pages for concurrent (v)CPUs faulting on the same
> page.
> 
> While this works as expected when touching one page-table leaf at a
> time, this can lead to difficult situations when mapping larger ranges.
> Indeed, a large map operation can fail in the middle if an existing
> mapping is found in the range, even if it has compatible attributes,
> hence leaving only half of the range mapped.

I'm curious of when this can happen. We normally map a single leaf at
a time, and we don't have a way to map multiple leaves at once: we
either use the VMA base size or try to upgrade it to a THP, but the
result is always a single leaf entry. What changed?

> To avoid having to deal with such failures in the caller, don't
> interrupt the map operation when hitting existing PTEs, but make sure to
> still return -EAGAIN so that user_mem_abort() can mark the page dirty
> when needed.

I don't follow you here: if you return -EAGAIN for a writable mapping,
we don't account for the page to be dirty on the assumption that
nothing has been mapped. But if there is a way to map more than a
single entry and to get -EAGAIN at the same time, then we're bound to
lose data on page eviction.

Can you shed some light on this?

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.