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

Mike Rapoport posted 5 patches 1 week, 1 day ago
There is a newer version of this series
[PATCH 3/5] mm: introduce VM_FAULT_UFFD_MINOR fault reason
Posted by Mike Rapoport 1 week, 1 day ago 
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.

Suggested-by: David Hildenbrand (Red Hat) <david@kernel.org>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.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 (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 aaa21bb60f51..6dcb73b52bcc 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
Re: [PATCH 3/5] mm: introduce VM_FAULT_UFFD_MINOR fault reason
Posted by David Hildenbrand (Red Hat) 1 week ago 
On 11/23/25 11:27, 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.
> 
> Suggested-by: David Hildenbrand (Red Hat) <david@kernel.org>
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.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
Re: [PATCH 3/5] mm: introduce VM_FAULT_UFFD_MINOR fault reason
Posted by Mike Rapoport 1 week ago 
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@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.
> > 
> > Suggested-by: David Hildenbrand (Red Hat) <david@kernel.org>
> > Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.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.

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