[ofa-general] Re: [PATCH] mmu notifiers #v8

[Andrea Arcangeli]

On Mon, Mar 03, 2008 at 02:10:17PM +0100, Nick Piggin wrote:
> Is this just a GRU problem? Can't we just require them to take a ref
> on the page (IIRC Jack said GRU could be changed to more like a TLB
> model).

Yes, it's just a GRU problem, it tries to optimize performance by
calling follow_page only in the fast path, and fallbacks to
get_user_pages; put_page in the slow path. xpmem could also send the
message in _begin and wait the message in _end, to reduce the wait
time. But if you forge GRU to call get_user_pages only (like KVM
does), the _begin can be removed. In theory we could also optimize KVM
to use follow_page only if the pte is already established. I'm not
sure how much that is a worthwhile optimization though.

However note that Quadrics also had a callback before and one after,
so they may be using the callback before for similar
optimizations. But functionality-wise _end is the only required bit if
everyone takes refcounts like KVM and XPMEM do.