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.com
Signed-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>

1 files changed, 2 insertions, 0 deletions

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 84530876b2ae..b105a455124d 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4855,6 +4855,7 @@ out_mutex:
 	return ret;
 }
 
+#ifdef CONFIG_USERFAULTFD
 /*
  * Used by userfaultfd UFFDIO_COPY.  Based on mcopy_atomic_pte with
  * modifications for huge pages.
@@ -4985,6 +4986,7 @@ out_release_nounlock:
 	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,