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 >
