linux-kernel - Re: [PATCH 3/5] mm: introduce VM_FAULT_UFFD

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Message-ID: <aSR1NxBi9ZYoKhwT@kernel.org>
Date: Mon, 24 Nov 2025 17:09:43 +0200
From: Mike Rapoport <rppt@...nel.org>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
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>,
	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>,
	Nikita Kalyazin <kalyazin@...zon.com>,
	Paolo Bonzini <pbonzini@...hat.com>, Peter Xu <peterx@...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 3/5] mm: introduce VM_FAULT_UFFD_MINOR fault reason

On Mon, Nov 24, 2025 at 12:05:18PM +0100, David Hildenbrand (Red Hat) wrote:
> On 11/23/25 11:27, Mike Rapoport wrote:
> > From: "Mike Rapoport (Microsoft)" <rppt@...nel.org>
> > 
> > When a VMA is registered with userfaulfd in minor mode, its ->fault()
> > method should check if a folio exists in the page cache and if yes
> > ->fault() should call handle_userfault(VM_UFFD_MISSING).
> > 
> > Instead of calling handle_userfault() directly from a specific ->fault()
> > implementation introduce new fault reason VM_FAULT_UFFD_MINOR that will
> > notify the core page fault handler that it should call
> > handle_userfaultfd(VM_UFFD_MISSING) to complete a page fault.
> > 
> > Replace a call to handle_userfault(VM_UFFD_MISSING) in shmem and use the
> > new VM_FAULT_UFFD_MINOR there instead.
> > 
> > Suggested-by: David Hildenbrand (Red Hat) <david@...nel.org>
> > Signed-off-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> > ---
> >   include/linux/mm_types.h | 3 +++
> >   mm/memory.c              | 2 ++
> >   mm/shmem.c               | 2 +-
> >   3 files changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 90e5790c318f..eb135369940f 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -1523,6 +1523,8 @@ typedef __bitwise unsigned int vm_fault_t;
> >    *				fsync() to complete (for synchronous page faults
> >    *				in DAX)
> >    * @VM_FAULT_COMPLETED:		->fault completed, meanwhile mmap lock released
> > + * @VM_FAULT_UFFD_MINOR:	->fault did not modify page tables and needs
> > + *				handle_userfault(VM_UFFD_MINOR) to complete
> >    * @VM_FAULT_HINDEX_MASK:	mask HINDEX value
> >    *
> >    */
> > @@ -1540,6 +1542,7 @@ enum vm_fault_reason {
> >   	VM_FAULT_DONE_COW       = (__force vm_fault_t)0x001000,
> >   	VM_FAULT_NEEDDSYNC      = (__force vm_fault_t)0x002000,
> >   	VM_FAULT_COMPLETED      = (__force vm_fault_t)0x004000,
> > +	VM_FAULT_UFFD_MINOR	= (__force vm_fault_t)0x008000,
> >   	VM_FAULT_HINDEX_MASK    = (__force vm_fault_t)0x0f0000,
> >   };
> > diff --git a/mm/memory.c b/mm/memory.c
> > index b59ae7ce42eb..94acbac8cefb 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -5279,6 +5279,8 @@ static vm_fault_t __do_fault(struct vm_fault *vmf)
> >   	}
> >   	ret = vma->vm_ops->fault(vmf);
> > +	if (unlikely(ret & VM_FAULT_UFFD_MINOR))
> > +		return handle_userfault(vmf, VM_UFFD_MINOR);
> 
> If we could define VM_FAULT_UFFD_MINOR to be 0 without USERFAULTFD, we could
> optimize that check out completely on such configs.
 
It will be a bit ugly, but we can :)
 
> -- 
> Cheers
> 
> David

-- 
Sincerely yours,
Mike.