Is this something cheri could use too?

In CheriBSD we have a somewhat different mechanism where we embed the tags in struct swblk due to a hesitance to add more allocation to the swap path. That's not great for memory usage since it's quite a lot of space and it should be fairly compressible in most cases. I'd expect MTE to be before more compressible given it's often going to be runs of the same value. I think we could adapt this code to add CHERI tags, likely in parallel, though maybe with a shared scan loop.

In D55957#1314750, @brooks wrote:

In CheriBSD we have a somewhat different mechanism where we embed the tags in struct swblk due to a hesitance to add more allocation to the swap path. That's not great for memory usage since it's quite a lot of space and it should be fairly compressible in most cases. I'd expect MTE to be before more compressible given it's often going to be runs of the same value. I think we could adapt this code to add CHERI tags, likely in parallel, though maybe with a shared scan loop.

struct swblk is a fixed size, so compressing tags there won't affect memory usage. I put the tags in a separate allocation as I expect the common case is no MTE in the immediate future, so the extra space will be wasted. In CheriBSD it's different as it's more likely there will be tags in a given page.

I didn't want to compress the tags yet as it's more likely the code would be incorrect as it will likely need to allocate a number of smaller fixed-sized regions to store them in. I don't have any numbers on the size of these regions as we still need userspace support, e.g. ASAN with MTE.