Re: arm64 MTE tag storage reuse - alternatives to MIGRATE_CMA

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi,

On Tue, Feb 20, 2024 at 03:07:22PM +0100, David Hildenbrand wrote:
> > > 
> > > 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.

Indeed.

> 
> > 
> > > 
> > > > 
> > > > 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.

So I guess instead of ZONE_DEVICE I should try to use arch_add_memory()
directly? That has the limitation that it cannot be used by a driver
(symbol not exported to modules).

Thanks,
Alex