userfaultfd: hugetlbfs: only compile UFFD helpers if config enabled

For background, mm/userfaultfd.c provides a general mcopy_atomic
implementation.  But some types of memory (i.e., hugetlb and shmem) need
a slightly different implementation, so they provide their own helpers
for this.  In other words, userfaultfd is the only caller of these
functions.

This patch achieves two things:

1. Don't spend time compiling code which will end up never being
   referenced anyway (a small build time optimization).

2. In patches later in this series, we extend the signature of these
   helpers with UFFD-specific state (a mode enumeration).  Once this
   happens, we *have to* either not compile the helpers, or
   unconditionally define the UFFD-only state (which seems messier to me).
   This includes the declarations in the headers, as otherwise they'd
   yield warnings about implicitly defining the type of those arguments.

Link: https://lkml.kernel.org/r/20210301222728.176417-4-axelrasmussen@google.comSigned-off-by: Axel Rasmussen <axelrasmussen@google.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Cc: Adam Ruprecht <ruprecht@google.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Cannon Matthews <cannonmatthews@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Chinwen Chang <chinwen.chang@mediatek.com>
Cc: David Rientjes <rientjes@google.com>
Cc: "Dr . David Alan Gilbert" <dgilbert@redhat.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: "Michal Koutn" <mkoutny@suse.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Mina Almasry <almasrymina@google.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Oliver Upton <oupton@google.com>
Cc: Shaohua Li <shli@fb.com>
Cc: Shawn Anastasio <shawn@anastas.io>
Cc: Steven Price <steven.price@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/hugetlb.h

@@ -134,11 +134,13 @@ void hugetlb_show_meminfo(void);
 unsigned long hugetlb_total_pages(void);
 vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 			unsigned long address, unsigned int flags);
+#ifdef CONFIG_USERFAULTFD
 int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm, pte_t *dst_pte,
 				struct vm_area_struct *dst_vma,
 				unsigned long dst_addr,
 				unsigned long src_addr,
 				struct page **pagep);
+#endif /* CONFIG_USERFAULTFD */
 bool hugetlb_reserve_pages(struct inode *inode, long from, long to,
 						struct vm_area_struct *vma,
 						vm_flags_t vm_flags);
@@ -310,6 +312,7 @@ static inline void hugetlb_free_pgd_range(struct mmu_gather *tlb,
 	BUG();
 }
 
+#ifdef CONFIG_USERFAULTFD
 static inline int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 						pte_t *dst_pte,
 						struct vm_area_struct *dst_vma,
@@ -320,6 +323,7 @@ static inline int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 	BUG();
 	return 0;
 }
+#endif /* CONFIG_USERFAULTFD */
 
 static inline pte_t *huge_pte_offset(struct mm_struct *mm, unsigned long addr,
 					unsigned long sz)

mm/hugetlb.c

@@ -4855,6 +4855,7 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	return ret;
 }
 
+#ifdef CONFIG_USERFAULTFD
 /*
  * Used by userfaultfd UFFDIO_COPY.  Based on mcopy_atomic_pte with
  * modifications for huge pages.
@@ -4985,6 +4986,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 	put_page(page);
 	goto out;
 }
+#endif /* CONFIG_USERFAULTFD */
 
 static void record_subpages_vmas(struct page *page, struct vm_area_struct *vma,
 				 int refs, struct page **pages,