On Tue, Oct 30, 2018 at 09:41:13PM -0700, Andy Lutomirski wrote: > To clarify some of this thread, I think that the fact that rare_write > uses an mm_struct and alias mappings under the hood should be > completely invisible to users of the API. No one should ever be > handed a writable pointer to rare_write memory (except perhaps during > bootup or when initializing a large complex data structure that will > be rare_write but isn't yet, e.g. the policy db). Being able to use pointers would make it far easier to do atomics and other things though. > For example, there could easily be architectures where having a > writable alias is problematic. Mostly we'd just have to be careful of cache aliases, alignment should be able to sort that I think. > If you have multiple pools and one mm_struct per pool, you'll need a > way to find the mm_struct from a given allocation. Or keep track of it externally. For example by context. If you modify page-tables you pick the page-table pool, if you modify selinux state, you pick the selinux pool. > Regardless of how the mm_structs are set up, changing rare_write > memory to normal memory or vice versa will require a global TLB flush > (all ASIDs and global pages) on all CPUs, so having extra mm_structs > doesn't seem to buy much. The way I understand it, the point is that if you stick page-tables and selinux state in different pools, a stray write in one will never affect the other.
