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

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Message-ID: <ef739802-1b53-4baa-81e4-dcc674d7c1c6@kernel.org>
Date: Mon, 24 Nov 2025 12:05:18 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Mike Rapoport <rppt@...nel.org>, linux-mm@...ck.org
Cc: 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 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.

-- 
Cheers

David