Re: [PATCH RFC 07/17] userfaultfd: introduce vm_uffd_ops

[Mike Rapoport <rppt@kernel.org>]

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@kernel.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@kernel.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.