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

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* 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
>