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10:02]: > On Wed, Oct 01, 2025 at 04:39:50PM +0200, David Hildenbrand wrote: > > On 01.10.25 16:35, Peter Xu wrote: > > > On Wed, Oct 01, 2025 at 03:58:14PM +0200, David Hildenbrand wrote: > > > > > > > > I briefly wondered whether we could use actual UFFD_FEATURE= _* here, but they > > > > > > > > are rather unsuited for this case here (e.g., different fea= ture flags for > > > > > > > > hugetlb support/shmem support etc). I think this supports the need for a code clean up before applying an API that generalizes it? I would expect the uffd code that needs the same uffd_feature would logically have the same uffd flags for the uffd_ops, but that's not the case here? Is this because uffd_feature !=3D UFFD_FEATURE_* ... or are the internal UFFD_FEATURE_* not the same thing? > > > > > > > >=20 > > > > > > > > But reading "uffd_ioctls" below, can't we derive the suitab= le vma flags from > > > > > > > > the supported ioctls? > > > > > > > >=20 > > > > > > > > _UFFDIO_COPY | _UFDIO_ZEROPAGE -> VM_UFFD_MISSING > > > > > > > > _UFFDIO_WRITEPROTECT -> VM_UFFD_WP > > > > > > > > _UFFDIO_CONTINUE -> VM_UFFD_MINOR > > > > > > >=20 > > > > > > > Yes we can deduce that, but it'll be unclear then when one st= ares at a > > > > > > > bunch of ioctls and cannot easily digest the modes the memory= type > > > > > > > supports. Here, the modes should be the most straightforward= way to > > > > > > > describe the capability of a memory type. > > > > > >=20 > > > > > > I rather dislike the current split approach between vm-flags an= d ioctls. > > > > > >=20 > > > > > > I briefly thought about abstracting it for internal purposes fu= rther and > > > > > > just have some internal backend ("memory type") flags. > > > > > >=20 > > > > > > UFFD_BACKEND_FEAT_MISSING -> _UFFDIO_COPY and VM_UFFD_MISSING > > > > > > UFFD_BACKEND_FEAT_ZEROPAGE -> _UFDIO_ZEROPAGE > > > > > > UFFD_BACKEND_FEAT_WP -> _UFFDIO_WRITEPROTECT and VM_UFFD_WP > > > > > > UFFD_BACKEND_FEAT_MINOR -> _UFFDIO_CONTINUE and VM_UFFD_MINOR > > > > > > UFFD_BACKEND_FEAT_POISON -> _UFFDIO_POISON > > > > >=20 > > > > > This layer of mapping can be helpful to some, but maybe confusing= to > > > > > others.. who is familiar with existing userfaultfd definitions. > > > > >=20 > > > >=20 > > > > Just wondering, is this confusing to you, and if so, which part? > > > >=20 > > > > To me it makes perfect sense and cleans up this API and not have to= sets of > > > > flags that are somehow interlinked. > > >=20 > > > It adds the extra layer of mapping that will only be used in vm_uffd_= ops > > > and the helper that will consume it. > >=20 > > Agreed, while making the API cleaner. I don't easily see what's confusi= ng > > about that, though. >=20 > It will introduce another set of userfaultfd features, making it hard to > say what is the difference between the new set and UFFD_FEATURE_*. If it's not using UFFD_FEATURE_ defines, then please don't use uffd_feature for it in the uffd_ops. That seems like a recipe for confusion. >=20 > >=20 > > I think it can be done with a handful of LOC and avoid having to use VM= _ > > flags in this API. >=20 > I waited for a few days, unfortunately we didn't get a second opinion. Sorry, been pretty busy here. If we can avoid the flags/features, then I'd rather that (the derived from uffd_ops =3D=3D NULL for support). We can always add something else later. If we have to have a feature/flag setting, then please avoid using uffd_feature unless we use it with UFFD_FEATURE_ - which I think, we've ruled out? Thanks, Liam