With large folios in place, we'd likely want to investigate not working on
individual pages, but on (possibly large) folios instead.
Yes, that would be interesting. Since the backend has no way of controlling
what tag storage page will be needed for tags, and subsequently dropped
from the cache, we would have to figure out what to do if one of the pages
that is part of a large folio is dropped. The easiest solution that I can
see is to remove the entire folio from the cleancache, but that would mean
also dropping the rest of the pages from the folio unnecessarily.
Right, but likely that won't be an issue. Things get interesting when
thinking about an efficient allocation approach.
I believe this is a very good fit for tag storage reuse, because it allows
tag storage to be allocated even in atomic contexts, which enables MTE in
the kernel. As a bonus, all of the changes to MM from the current approach
wouldn't be needed, as tag storage allocation can be handled entirely in
set_ptes_at(), copy_*highpage() or arch_swap_restore().
Is this a viable approach that would be upstreamable? Are there other
solutions that I haven't considered? I'm very much open to any alternatives
that would make tag storage reuse viable.
As raised recently, I had similar ideas with something like virtio-mem in
the past (wanted to call it virtio-tmem back then), but didn't have time to
look into it yet.
I considered both, using special device memory as "cleancache" backend, and
using it as backend storage for something similar to zswap. We would not
need a memmap/"struct page" for that special device memory, which reduces
memory overhead and makes "adding more memory" a more reliable operation.
Hm... this might not work with tag storage memory, the kernel needs to
perform cache maintenance on the memory when it transitions to and from
storing tags and storing data, so the memory must be mapped by the kernel.
The direct map will definitely be required I think (copy in/out data).
But memmap for tag memory will likely not be required. Of course, it
depends how to manage tag storage. Likely we have to store some
metadata, hopefully we can avoid the full memmap and just use something
else.
[...]
Similar to virtio-mem, there are ways for the hypervisor to request changes
to the memory consumption of a device (setting the requested size). So when
requested to consume less, clean pagecache pages can be dropped and the
memory can be handed back to the hypervisor.
Of course, likely we would want to consider using "slower" memory in the
hypervisor to back such a device.
I'm not sure how useful that will be with tag storage reuse. KVM must
assume that **all** the memory that the guest uses is tagged and it needs
tag storage allocated (it's a known architectural limitation), so that will
leave even less tag storage memory to distribute between the host and the
guest(s).
Yes, I don't think this applies to tag storage.
Adding to that, at the moment Android is going to be the major (only?) user
of tag storage reuse, and as far as I know pKVM is more restrictive with
regards to the emulated devices and the memory that is shared between
guests and the host.
Right, what I described here does not have overlap with tag storage
besides requiring similar (cleancache) hooks.
--
Cheers,
David / dhildenb
