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

Liam R. Howlett
2025-11-26T15:19:40Z

* Mike Rapoport <rppt@kernel.org> [251125 13:39]:
> 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>

Same nit as David, but the rest looks good.

> ---
> 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" }, \
>
> 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)))

I have the same concern as David here with adding instructions to the
faults that are not UFFD_MINOR.. I suspect the compiler will remove the
statement completely when UFFD is disabled (and thus ret & 0 in the
check), but it might be worth looking at this closer in the case where
uffd is enabled? It won't be as clean looking but might make the
assembly better.

> 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
>