mm: restructure memfd code

With the addition of memfd hugetlbfs support, we now have the situation
where memfd depends on TMPFS -or- HUGETLBFS.  Previously, memfd was only
supported on tmpfs, so it made sense that the code resided in shmem.c.
In the current code, memfd is only functional if TMPFS is defined.  If
HUGETLFS is defined and TMPFS is not defined, then memfd functionality
will not be available for hugetlbfs.  This does not cause BUGs, just a
lack of potentially desired functionality.

Code is restructured in the following way:
- include/linux/memfd.h is a new file containing memfd specific
  definitions previously contained in shmem_fs.h.
- mm/memfd.c is a new file containing memfd specific code previously
  contained in shmem.c.
- memfd specific code is removed from shmem_fs.h and shmem.c.
- A new config option MEMFD_CREATE is added that is defined if TMPFS
  or HUGETLBFS is defined.

No functional changes are made to the code: restructuring only.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Mike Kravetz <[email protected]>
Reviewed-by: Khalid Aziz <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: David Herrmann <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Marc-Andr Lureau <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Michal Hocko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

fs/Kconfig

@@ -203,6 +203,9 @@ config HUGETLBFS
 config HUGETLB_PAGE
 	def_bool HUGETLBFS
 
+config MEMFD_CREATE
+	def_bool TMPFS || HUGETLBFS
+
 config ARCH_HAS_GIGANTIC_PAGE
 	bool
 

fs/fcntl.c

@@ -23,7 +23,7 @@
 #include <linux/rcupdate.h>
 #include <linux/pid_namespace.h>
 #include <linux/user_namespace.h>
-#include <linux/shmem_fs.h>
+#include <linux/memfd.h>
 #include <linux/compat.h>
 
 #include <linux/poll.h>

include/linux/memfd.h

@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __LINUX_MEMFD_H
+#define __LINUX_MEMFD_H
+
+#include <linux/file.h>
+
+#ifdef CONFIG_MEMFD_CREATE
+extern long memfd_fcntl(struct file *file, unsigned int cmd, unsigned long arg);
+#else
+static inline long memfd_fcntl(struct file *f, unsigned int c, unsigned long a)
+{
+	return -EINVAL;
+}
+#endif
+
+#endif /* __LINUX_MEMFD_H */

include/linux/shmem_fs.h

@@ -110,19 +110,6 @@ static inline bool shmem_file(struct file *file)
 extern bool shmem_charge(struct inode *inode, long pages);
 extern void shmem_uncharge(struct inode *inode, long pages);
 
-#ifdef CONFIG_TMPFS
-
-extern long memfd_fcntl(struct file *file, unsigned int cmd, unsigned long arg);
-
-#else
-
-static inline long memfd_fcntl(struct file *f, unsigned int c, unsigned long a)
-{
-	return -EINVAL;
-}
-
-#endif
-
 #ifdef CONFIG_TRANSPARENT_HUGE_PAGECACHE
 extern bool shmem_huge_enabled(struct vm_area_struct *vma);
 #else

mm/Makefile

@@ -105,3 +105,4 @@ obj-$(CONFIG_DEBUG_PAGE_REF) += debug_page_ref.o
 obj-$(CONFIG_HARDENED_USERCOPY) += usercopy.o
 obj-$(CONFIG_PERCPU_STATS) += percpu-stats.o
 obj-$(CONFIG_HMM) += hmm.o
+obj-$(CONFIG_MEMFD_CREATE) += memfd.o