On 30.01.25 07:11, Alistair Popple wrote:
On Wed, Jan 29, 2025 at 12:54:02PM +0100, David Hildenbrand wrote:
We require a writable PTE and only support anonymous folio: we can only
have exactly one PTE pointing at that page, which we can just lookup
using a folio walk, avoiding the rmap walk and the anon VMA lock.
So let's stop doing an rmap walk and perform a folio walk instead, so we
can easily just modify a single PTE and avoid relying on rmap/mapcounts.
We now effectively work on a single PTE instead of multiple PTEs of
a large folio, allowing for conversion of individual PTEs from
non-exclusive to device-exclusive -- note that the other way always
worked on single PTEs.
We can drop the MMU_NOTIFY_EXCLUSIVE MMU notifier call and document why
that is not required: GUP will already take care of the
MMU_NOTIFY_EXCLUSIVE call if required (there is already a device-exclusive
entry) when not finding a present PTE and having to trigger a fault and
ending up in remove_device_exclusive_entry().
I will have to look at this a bit more closely tomorrow but this doesn't seem
right to me. We may be transitioning from a present PTE (ie. a writable
anonymous mapping) to a non-present PTE (ie. a device-exclusive entry) and
therefore any secondary processors (eg. other GPUs, iommus, etc.) will need to
update their copies of the PTE. So I think the notifier call is needed.
Then it is all very confusing:
"MMU_NOTIFY_EXCLUSIVE: to signal a device driver that the device will no
longer have exclusive access to the page."
That's simply not true in the scenario you describe, because nobody had
exclusive access.
But what you are saying is, that we need to inform others (e.g., KVM)
that we are converting it to a device-exclusive entry, such that they
stop accessing it.
That makes sense to me (and the cleanup patch in the cleanup series
would have to go as well to prevent the livelock).
So we would have to update the documentation of MMU_NOTIFY_EXCLUSIVE
that it is also trigger on conversion from non-exclusive to exclusive.
Does that make sense?
Thanks!
--
Cheers,
David / dhildenb
