linux-kernel - Re: [PATCH v3 1/4] mm: Introduce vm_uffd

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Message-ID: <cq3zcvnajs55zr7cplf5oxxjoh54fb7tvo23hehd5dmh4atvum@6274mneik6hu>
Date: Mon, 6 Oct 2025 15:06:39 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Peter Xu <peterx@...hat.com>
Cc: David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        James Houghton <jthoughton@...gle.com>,
        Nikita Kalyazin <kalyazin@...zon.com>,
        Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
        Ujwal Kundur <ujwal.kundur@...il.com>, Mike Rapoport <rppt@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>, Michal Hocko <mhocko@...e.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Oscar Salvador <osalvador@...e.de>, Hugh Dickins <hughd@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v3 1/4] mm: Introduce vm_uffd_ops API

* Peter Xu <peterx@...hat.com> [251003 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 feature 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 != UFFD_FEATURE_* ... or are the internal
UFFD_FEATURE_* not the same thing?

> > > > > > > > 
> > > > > > > > But reading "uffd_ioctls" below, can't we derive the suitable vma flags from
> > > > > > > > the supported ioctls?
> > > > > > > > 
> > > > > > > > _UFFDIO_COPY | _UFDIO_ZEROPAGE -> VM_UFFD_MISSING
> > > > > > > > _UFFDIO_WRITEPROTECT -> VM_UFFD_WP
> > > > > > > > _UFFDIO_CONTINUE -> VM_UFFD_MINOR
> > > > > > > 
> > > > > > > Yes we can deduce that, but it'll be unclear then when one stares 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.
> > > > > > 
> > > > > > I rather dislike the current split approach between vm-flags and ioctls.
> > > > > > 
> > > > > > I briefly thought about abstracting it for internal purposes further and
> > > > > > just have some internal backend ("memory type") flags.
> > > > > > 
> > > > > > 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
> > > > > 
> > > > > This layer of mapping can be helpful to some, but maybe confusing to
> > > > > others.. who is familiar with existing userfaultfd definitions.
> > > > > 
> > > > 
> > > > Just wondering, is this confusing to you, and if so, which part?
> > > > 
> > > > To me it makes perfect sense and cleans up this API and not have to sets of
> > > > flags that are somehow interlinked.
> > > 
> > > It adds the extra layer of mapping that will only be used in vm_uffd_ops
> > > and the helper that will consume it.
> > 
> > Agreed, while making the API cleaner. I don't easily see what's confusing
> > about that, though.
> 
> 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.

> 
> > 
> > I think it can be done with a handful of LOC and avoid having to use VM_
> > flags in this API.
> 
> 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 == 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