On Thu, Apr 16, 2026 at 08:32:19PM +0200, David Hildenbrand (Arm) wrote: > On 4/16/26 15:49, Kiryl Shutsemau wrote: > > On Tue, Apr 14, 2026 at 06:10:44PM +0100, Kiryl Shutsemau wrote: > >> On Tue, Apr 14, 2026 at 05:37:50PM +0200, David Hildenbrand (Arm) wrote: > >>> > >>> I would rather tackle this from the other direction: it's another form > >>> of protection (like WP), not really a "minor" mode. > >>> > >>> Could we add a UFFDIO_REGISTER_MODE_RWP (or however we would call it) > >>> and support it for anon+shmem, avoiding the zapping for shmem completely? > >> > >> I like this idea. > >> > >> It should be functionally equivalent, but your interface idea fits > >> better with the rest. > >> > >> Thanks! Will give it a try. > > > > Here is an updated version: > > > > https://git.kernel.org/pub/scm/linux/kernel/git/kas/linux.git/log/?h=uffd/rfc-v2 > > > > will post after -rc1 is tagged. > > > > I like it more. It got substantially cleaner. > > I don't have time to look into the details just yet, but my thinking was > that > > a) It would avoid the zap+refault
Yep. > b) We could reuse the uffd-wp PTE bit + marker to indicate/remember the > protection, making it co-exist with NUMA hinting naturally. > > b) obviously means that we cannot use uffd-wp and uffd-rwp at the same > time in the same uffd area. I guess that should be acceptable for the > use cases we you should have in mind? I took a different path: I still use PROT_NONE PTEs, so it cannot co-exist with NUMA balancing [fully], but WP + RWP should be fine. I need to add a test for this. I didn't give up on NUMA balancing completely. task_numa_fault() is called on RWP fault. So it should help scheduler decisions somewhat. I think an RWP user might want to use WP too. Do you see this trade-off as reasonable? > But I also haven't taken a closer look at this patch set, whether you > would already be using a PTE bit somehow (I suspect not :) ) No. I didn't want to allocate a new bit or invent some arch-specific trick for this. This functionality is available everywhere where PAGE_NONE exists. -- Kiryl Shutsemau / Kirill A. Shutemov
