Re: [PATCH v2 3/5] mm: introduce VM_FAULT_UFFD_MINOR fault reason

[Nikita Kalyazin <kalyazin@amazon.com>]

On 25/11/2025 18:38, Mike Rapoport wrote:
> From: "Mike Rapoport (Microsoft)" <rppt@kernel.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.
> 
> For configurations that don't enable CONFIG_USERFAULTFD,
> VM_FAULT_UFFD_MINOR is set to 0.
> 
> Suggested-by: David Hildenbrand (Red Hat) <david@kernel.org>
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> ---
>   include/linux/mm_types.h | 10 +++++++++-
>   mm/memory.c              |  2 ++
>   mm/shmem.c               |  2 +-
>   3 files changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 90e5790c318f..df71b057111b 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 +1542,11 @@ 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,
> +#ifdef CONFIG_USERFAULTFD
> +       VM_FAULT_UFFD_MINOR     = (__force vm_fault_t)0x008000,
> +#else
> +       VM_FAULT_UFFD_MINOR     = (__force vm_fault_t)0x000000,
> +#endif
>          VM_FAULT_HINDEX_MASK    = (__force vm_fault_t)0x0f0000,
>   };
> 
> @@ -1564,7 +1571,8 @@ enum vm_fault_reason {
>          { VM_FAULT_FALLBACK,            "FALLBACK" },   \
>          { VM_FAULT_DONE_COW,            "DONE_COW" },   \
>          { VM_FAULT_NEEDDSYNC,           "NEEDDSYNC" },  \
> -       { VM_FAULT_COMPLETED,           "COMPLETED" }
> +       { VM_FAULT_COMPLETED,           "COMPLETED" },  \
> +       { VM_FAULT_UFFD_MINOR,          "UFFD_MINOR" }, \

It looks like we have to keep the last element comma-less, otherwise I'm 
seeing compile errors somewhere in fs/dax.c.

> 
>   struct vm_special_mapping {
>          const char *name;       /* The name, e.g. "[vdso]". */
> 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 (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY |
>                              VM_FAULT_DONE_COW)))
>                  return ret;
> diff --git a/mm/shmem.c b/mm/shmem.c
> index e16c7c8c3e1e..a9a31c0b5979 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2461,7 +2461,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
>          if (folio && vma && userfaultfd_minor(vma)) {
>                  if (!xa_is_value(folio))
>                          folio_put(folio);
> -               *fault_type = handle_userfault(vmf, VM_UFFD_MINOR);
> +               *fault_type = VM_FAULT_UFFD_MINOR;
>                  return 0;
>          }
> 
> --
> 2.50.1
>