zpl_super: prefer "new" mount API when available

This API has been available since kernel 5.2, and having it available
(almost) everywhere should give us a lot more flexibility for mount
management in the future.

Sponsored-by: TrueNAS
Reviewed-by: Tony Hutter <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Rob Norris <[email protected]>
Closes #18260

Author: robn

config/kernel-fs-context.m4

@@ -0,0 +1,33 @@
+dnl # SPDX-License-Identifier: CDDL-1.0
+dnl #
+dnl # 2.6.38 API change
+dnl # The .get_sb callback has been replaced by a .mount callback
+dnl # in the file_system_type structure.
+dnl #
+dnl # 5.2 API change
+dnl # The new fs_context-based filesystem API is introduced, with the old
+dnl # one (via file_system_type.mount) preserved as a compatibility shim.
+dnl #
+dnl # 7.0 API change
+dnl # Compatibility shim removed, so all callers must go through the mount API.
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_SRC_FS_CONTEXT], [
+	ZFS_LINUX_TEST_SRC([fs_context], [
+		#include <linux/fs.h>
+		#include <linux/fs_context.h>
+        ],[
+		static struct fs_context fs __attribute__ ((unused)) = { 0 };
+		static struct fs_context *fsp __attribute__ ((unused));
+		fsp = vfs_dup_fs_context(&fs);
+	])
+])
+
+AC_DEFUN([ZFS_AC_KERNEL_FS_CONTEXT], [
+        AC_MSG_CHECKING([whether fs_context exists])
+        ZFS_LINUX_TEST_RESULT([fs_context], [
+                AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_FS_CONTEXT, 1, [fs_context exists])
+        ],[
+		AC_MSG_RESULT(no)
+        ])
+])

config/kernel-fst-mount.m4

@@ -75,7 +75,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_SRC], [
 	ZFS_AC_KERNEL_SRC_DENTRY
 	ZFS_AC_KERNEL_SRC_TRUNCATE_SETSIZE
 	ZFS_AC_KERNEL_SRC_SECURITY_INODE
-	ZFS_AC_KERNEL_SRC_FST_MOUNT
+	ZFS_AC_KERNEL_SRC_FS_CONTEXT
 	ZFS_AC_KERNEL_SRC_SB_DYING
 	ZFS_AC_KERNEL_SRC_SET_NLINK
 	ZFS_AC_KERNEL_SRC_SGET
@@ -199,7 +199,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_RESULT], [
 	ZFS_AC_KERNEL_DENTRY
 	ZFS_AC_KERNEL_TRUNCATE_SETSIZE
 	ZFS_AC_KERNEL_SECURITY_INODE
-	ZFS_AC_KERNEL_FST_MOUNT
+	ZFS_AC_KERNEL_FS_CONTEXT
 	ZFS_AC_KERNEL_SB_DYING
 	ZFS_AC_KERNEL_SET_NLINK
 	ZFS_AC_KERNEL_SGET

config/kernel.m4

@@ -36,8 +36,7 @@
 #include <linux/iversion.h>
 #include <linux/version.h>
 #include <linux/vfs_compat.h>
-
-#ifndef HAVE_FST_MOUNT
+#ifdef HAVE_FS_CONTEXT
 #include <linux/fs_context.h>
 #endif
 
@@ -509,17 +508,15 @@ zpl_prune_sb(uint64_t nr_to_scan, void *arg)
 #endif
 }
 
-#ifndef HAVE_FST_MOUNT
+#ifdef HAVE_FS_CONTEXT
 /*
- * In kernel 7.0, the file_system_type->mount() and
- * super_operations->remount_fs() callbacks have been removed, requiring all
- * users to convert to the "new" fs_context-based mount API introduced in 5.2.
+ * Since kernel 5.2, the "new" fs_context-based mount API has been preferred
+ * over the traditional file_system_type->mount() and
+ * super_operations->remount_fs() callbacks, which were deprectate. In 7.0,
+ * those callbacks were removed.
  *
- * This is the simplest compatibility shim possible to adapt the fs_context
- * interface to the old-style calls. Although this interface exists in almost
- * all versions of Linux currently supported by OpenZFS, we only use it when
- * the kernel-provided shims are unavailable, to avoid bugs in these new shims
- * affecting all OpenZFS deployments.
+ * Currently, the old-style interface are the only ones we need, so this is
+ * a simple compatibility shim to adapt the new API to the old-style calls.
  */
 static int
 zpl_parse_monolithic(struct fs_context *fc, void *data)
@@ -577,7 +574,7 @@ const struct super_operations zpl_super_operations = {
 	.put_super		= zpl_put_super,
 	.sync_fs		= zpl_sync_fs,
 	.statfs			= zpl_statfs,
-#ifdef HAVE_FST_MOUNT
+#ifndef HAVE_FS_CONTEXT
 	.remount_fs		= zpl_remount_fs,
 #endif
 	.show_devname		= zpl_show_devname,
@@ -622,10 +619,10 @@ struct file_system_type zpl_fs_type = {
 #else
 	.fs_flags		= FS_USERNS_MOUNT,
 #endif
-#ifdef HAVE_FST_MOUNT
-	.mount			= zpl_mount,
-#else
+#ifdef HAVE_FS_CONTEXT
 	.init_fs_context	= zpl_init_fs_context,
+#else
+	.mount			= zpl_mount,
 #endif
 	.kill_sb		= zpl_kill_sb,
 };