Re: [PATCH v1 04/12] mm/rmap: implement make_device_exclusive() using folio_walk instead of rmap walk

[Simona Vetter <simona.vetter@ffwll.ch>]

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