On Thu, Jan 30, 2025 at 04:59:16PM +0100, David Hildenbrand wrote: > > > > > > Note that the PTE is > > > > > > always writable, and we can always create a writable-device-exclusive > > > > > > entry. > > > > > > > > > > > > With this change, device-exclusive is fully compatible with THPs / > > > > > > large folios. We still require PMD-sized THPs to get PTE-mapped, and > > > > > > supporting PMD-mapped THP (without the PTE-remapping) is a different > > > > > > endeavour that might not be worth it at this point. > > > > > > > > I'm not sure we actually want hugepages for device exclusive, since it has > > > > an impact on what's allowed and what not. If we only ever do 4k entries > > > > then userspace can assume that as long atomics are separated by a 4k page > > > > there's no issue when both the gpu and cpu hammer on them. If we try to > > > > keep thp entries then suddenly a workload that worked before will result > > > > in endless ping-pong between gpu and cpu because the separate atomic > > > > counters (or whatever) now all sit in the same 2m page. > > > > > > Agreed. And the conversion + mapping into the device gets trickier. > > > > > > > > > > > So going with thp might result in userspace having to spread out atomics > > > > even more, which is just wasting memory and not saving any tlb entries > > > > since often you don't need that many. > > > > > > > > tldr; I think not supporting thp entries for device exclusive is a > > > > feature, not a bug. > > > > > > So, you agree with my "different endeavour that might not be worth it" > > > statement? > > > > Yes. > > > > Well I think we should go further and clearly document that we > > intentionally return split pages. Because it's part of the uapi contract > > with users of all this. > > Yes, see my reply to patch #3/ Ack. > > And if someone needs pmd entries for performance or whatever, we need two > > things: > > > > a) userspace must mmap that memory as hugepage memory, to clearly signal > > the promise that atomics are split up on hugepage sizes and not just page > > size > > > > b) we need to extend make_device_exclusive and drivers to handle the > > hugetlb folio case > > > > I think thp is simply not going to work here, it's impossible (without > > potentially causing fault storms) to figure out what userspace might want. > > Right, I added a link to this discussion in the patch. Thanks, Sima -- Simona Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
