On Mon, 19 Jul 2021 11:47:24 +0100,
Quentin Perret [off-list ref] 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.

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