> - gleb asks: don't we want to avoid the system call when possible? > - patrick: a single syscall can be/is cheaper than a reg cache > lookup in user space
This doesn't really make sense -- syscall + cache lookup in kernel is "obviously" more expensive than cache lookup in userspace with no context switch (I don't see any tricks the kernel can do that make the cache lookup cheaper there). However the solution I proposed a long time ago (when Pete Wyckoff originally did his work on having the kernel track this -- and as a side note, it's not clear to me whether MMU notifiers really help what Pete did) is for userspace to provide a pointer to a flag when registering memory with the kernel, and then the kernel can mark the flag if the mapping changes -- ie keep the userspace cache but have the kernel manage invalidation "perfectly" without any malloc hooks. - R.
