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Message-ID: <aYhh2XzyFsJbohll@kernel.org>
Date: Sun, 8 Feb 2026 12:13:45 +0200
From: Mike Rapoport <rppt@...nel.org>
To: Peter Xu <peterx@...hat.com>
Cc: linux-mm@...ck.org, Andrea Arcangeli <aarcange@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Axel Rasmussen <axelrasmussen@...gle.com>,
Baolin Wang <baolin.wang@...ux.alibaba.com>,
David Hildenbrand <david@...hat.com>,
Hugh Dickins <hughd@...gle.com>,
James Houghton <jthoughton@...gle.com>,
"Liam R. Howlett" <Liam.Howlett@...cle.com>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
Michal Hocko <mhocko@...e.com>, Muchun Song <muchun.song@...ux.dev>,
Nikita Kalyazin <kalyazin@...zon.com>,
Oscar Salvador <osalvador@...e.de>,
Paolo Bonzini <pbonzini@...hat.com>,
Sean Christopherson <seanjc@...gle.com>,
Shuah Khan <shuah@...nel.org>,
Suren Baghdasaryan <surenb@...gle.com>,
Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
kvm@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH RFC 07/17] userfaultfd: introduce vm_uffd_ops
Hi Peter,
On Mon, Feb 02, 2026 at 04:36:40PM -0500, Peter Xu wrote:
> On Tue, Jan 27, 2026 at 09:29:26PM +0200, Mike Rapoport wrote:
> > From: "Mike Rapoport (Microsoft)" <rppt@...nel.org>
> >
> > Current userfaultfd implementation works only with memory managed by
> > core MM: anonymous, shmem and hugetlb.
> >
> > First, there is no fundamental reason to limit userfaultfd support only
> > to the core memory types and userfaults can be handled similarly to
> > regular page faults provided a VMA owner implements appropriate
> > callbacks.
> >
> > Second, historically various code paths were conditioned on
> > vma_is_anonymous(), vma_is_shmem() and is_vm_hugetlb_page() and some of
> > these conditions can be expressed as operations implemented by a
> > particular memory type.
> >
> > Introduce vm_uffd_ops extension to vm_operations_struct that will
> > delegate memory type specific operations to a VMA owner.
> >
> > Operations for anonymous memory are handled internally in userfaultfd
> > using anon_uffd_ops that implicitly assigned to anonymous VMAs.
> >
> > Start with a single operation, ->can_userfault() that will verify that a
> > VMA meets requirements for userfaultfd support at registration time.
> >
> > Implement that method for anonymous, shmem and hugetlb and move relevant
> > parts of vma_can_userfault() into the new callbacks.
> >
> > Signed-off-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> > ---
> > include/linux/mm.h | 5 +++++
> > include/linux/userfaultfd_k.h | 6 +++++
> > mm/hugetlb.c | 21 ++++++++++++++++++
> > mm/shmem.c | 23 ++++++++++++++++++++
> > mm/userfaultfd.c | 41 ++++++++++++++++++++++-------------
> > 5 files changed, 81 insertions(+), 15 deletions(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 15076261d0c2..3c2caff646c3 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -732,6 +732,8 @@ struct vm_fault {
> > */
> > };
> >
> > +struct vm_uffd_ops;
> > +
> > /*
> > * These are the virtual MM functions - opening of an area, closing and
> > * unmapping it (needed to keep files on disk up-to-date etc), pointer
> > @@ -817,6 +819,9 @@ struct vm_operations_struct {
> > struct page *(*find_normal_page)(struct vm_area_struct *vma,
> > unsigned long addr);
> > #endif /* CONFIG_FIND_NORMAL_PAGE */
> > +#ifdef CONFIG_USERFAULTFD
> > + const struct vm_uffd_ops *uffd_ops;
> > +#endif
> > };
> >
> > #ifdef CONFIG_NUMA_BALANCING
> > diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> > index a49cf750e803..56e85ab166c7 100644
> > --- a/include/linux/userfaultfd_k.h
> > +++ b/include/linux/userfaultfd_k.h
> > @@ -80,6 +80,12 @@ struct userfaultfd_ctx {
> >
> > extern vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason);
> >
> > +/* VMA userfaultfd operations */
> > +struct vm_uffd_ops {
> > + /* Checks if a VMA can support userfaultfd */
> > + bool (*can_userfault)(struct vm_area_struct *vma, vm_flags_t vm_flags);
> > +};
> > +
> > /* A combined operation mode + behavior flags. */
> > typedef unsigned int __bitwise uffd_flags_t;
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 51273baec9e5..909131910c43 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -4797,6 +4797,24 @@ static vm_fault_t hugetlb_vm_op_fault(struct vm_fault *vmf)
> > return 0;
> > }
> >
> > +#ifdef CONFIG_USERFAULTFD
> > +static bool hugetlb_can_userfault(struct vm_area_struct *vma,
> > + vm_flags_t vm_flags)
> > +{
> > + /*
> > + * If user requested uffd-wp but not enabled pte markers for
> > + * uffd-wp, then hugetlb is not supported.
> > + */
> > + if (!uffd_supports_wp_marker() && (vm_flags & VM_UFFD_WP))
> > + return false;
>
> IMHO we don't need to dup this for every vm_uffd_ops driver. It might be
> unnecessary to even make driver be aware how pte marker plays the role
> here, because pte markers are needed for all page cache file systems
> anyway. There should have no outliers. Instead we can just let
> can_userfault() report whether the driver generically supports userfaultfd,
> leaving the detail checks for core mm.
>
> I understand you wanted to also make anon to be a driver, so this line
> won't apply to anon. However IMHO anon is special enough so we can still
> make this in the generic path.
Well, the idea is to drop all vma_is*() in can_userfault(). And maybe
eventually in entire mm/userfaultfd.c
If all page cache filesystems need this, something like this should work,
right?
if (!uffd_supports_wp_marker() && (vma->vm_flags & VM_SHARED) &&
(vm_flags & VM_UFFD_WP))
return false;
--
Sincerely yours,
Mike.
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