Re: arm64 MTE tag storage reuse - alternatives to MIGRATE_CMA

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

Hi,

On Tue, Feb 20, 2024 at 05:16:26PM +0100, David Hildenbrand wrote:
> > > > > > 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).
> You can certainly start with something simple, and we can work on removing
> that memmap allocation later.
> 
> Maybe we have to expose new primitives in the context of such drivers.
> arch_add_memory() likely also doesn't do what you need.
> 
> I recall that we had a way of only messing with the direct map.
> 
> Last time I worked with that was in the context of memtrace
> (arch/powerpc/platforms/powernv/memtrace.c)
> 
> There, we call arch_create_linear_mapping()/arch_remove_linear_mapping().
> 
> ... and now my memory comes back: we never finished factoring out
> arch_create_linear_mapping/arch_remove_linear_mapping so they would be
> available on all architectures.
> 
> 
> Your driver will be very arm64 specific, so doing it in an arm64-special way
> might be good enough initially. For example, the arm64-core could detect
> that special memory region and just statically prepare the direct map and
> not expose the memory to the buddy/allocate a memmap. Similar to how we
> handle the crashkernel/kexec IIRC (we likely do not have a direct map for
> that, though; ).
> 
> [I was also wondering if we could simply dynamically map/unmap when required
> so you can just avoid creating the entire direct map; might bot be the best
> approach performance-wise, though]
> 
> There are a bunch of details to be sorted out, but I don't consider the
> directmap/memmap side of things a big problem.

Sounds reasonable, thank you for the feedback!

Thanks,
Alex

> 
> -- 
> Cheers,
> 
> David / dhildenb
>