fs: Remove internal old mount API code

Now that the last in-tree filesystem has been converted to the new mount
API, remove all legacy mount API code designed to handle un-converted
filesystems, and remove associated documentation as well.

(The code to handle the legacy mount(2) syscall from userspace is still
in place, of course.)

Tested with an allmodconfig build on x86_64, and a sanity check of an
old mount(2) syscall mount.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Link: https://patch.msgid.link/20251212174403.2882183-1-sandeen@redhat.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Eric Sandeen

Documentation/filesystems/locking.rst

@@ -177,7 +177,6 @@ prototypes::
 	int (*freeze_fs) (struct super_block *);
 	int (*unfreeze_fs) (struct super_block *);
 	int (*statfs) (struct dentry *, struct kstatfs *);
-	int (*remount_fs) (struct super_block *, int *, char *);
 	void (*umount_begin) (struct super_block *);
 	int (*show_options)(struct seq_file *, struct dentry *);
 	ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
@@ -201,7 +200,6 @@ sync_fs:		read
 freeze_fs:		write
 unfreeze_fs:		write
 statfs:			maybe(read)	(see below)
-remount_fs:		write
 umount_begin:		no
 show_options:		no		(namespace_sem)
 quota_read:		no		(see below)
@@ -226,22 +224,16 @@ file_system_type
 
 prototypes::
 
-	struct dentry *(*mount) (struct file_system_type *, int,
-		       const char *, void *);
 	void (*kill_sb) (struct super_block *);
 
 locking rules:
 
 =======		=========
 ops		may block
 =======		=========
-mount		yes
 kill_sb		yes
 =======		=========
 
-->mount() returns ERR_PTR or the root dentry; its superblock should be locked
-on return.
-
 ->kill_sb() takes a write-locked superblock, does all shutdown work on it,
 unlocks and drops the reference.
 

Documentation/filesystems/mount_api.rst

@@ -299,8 +299,6 @@ manage the filesystem context.  They are as follows:
      On success it should return 0.  In the case of an error, it should return
      a negative error code.
 
-     .. Note:: reconfigure is intended as a replacement for remount_fs.
-
 
 Filesystem context Security
 ===========================

Documentation/filesystems/porting.rst

@@ -448,11 +448,8 @@ a file off.
 
 **mandatory**
 
-->get_sb() is gone.  Switch to use of ->mount().  Typically it's just
-a matter of switching from calling ``get_sb_``... to ``mount_``... and changing
-the function type.  If you were doing it manually, just switch from setting
-->mnt_root to some pointer to returning that pointer.  On errors return
-ERR_PTR(...).
+->get_sb() and ->mount() are gone. Switch to using the new mount API. See
+Documentation/filesystems/mount_api.rst for more details.
 
 ---
 

Documentation/filesystems/vfs.rst

@@ -94,11 +94,9 @@ functions:
 
 The passed struct file_system_type describes your filesystem.  When a
 request is made to mount a filesystem onto a directory in your
-namespace, the VFS will call the appropriate mount() method for the
-specific filesystem.  New vfsmount referring to the tree returned by
-->mount() will be attached to the mountpoint, so that when pathname
-resolution reaches the mountpoint it will jump into the root of that
-vfsmount.
+namespace, the VFS will call the appropriate get_tree() method for the
+specific filesystem.  See Documentation/filesystems/mount_api.rst
+for more details.
 
 You can see all filesystems that are registered to the kernel in the
 file /proc/filesystems.
@@ -117,8 +115,6 @@ members are defined:
 		int fs_flags;
 		int (*init_fs_context)(struct fs_context *);
 		const struct fs_parameter_spec *parameters;
-		struct dentry *(*mount) (struct file_system_type *, int,
-			const char *, void *);
 		void (*kill_sb) (struct super_block *);
 		struct module *owner;
 		struct file_system_type * next;
@@ -151,10 +147,6 @@ members are defined:
 	'struct fs_parameter_spec'.
 	More info in Documentation/filesystems/mount_api.rst.
 
-``mount``
-	the method to call when a new instance of this filesystem should
-	be mounted
-
 ``kill_sb``
 	the method to call when an instance of this filesystem should be
 	shut down
@@ -173,45 +165,6 @@ members are defined:
   s_lock_key, s_umount_key, s_vfs_rename_key, s_writers_key,
   i_lock_key, i_mutex_key, invalidate_lock_key, i_mutex_dir_key: lockdep-specific
 
-The mount() method has the following arguments:
-
-``struct file_system_type *fs_type``
-	describes the filesystem, partly initialized by the specific
-	filesystem code
-
-``int flags``
-	mount flags
-
-``const char *dev_name``
-	the device name we are mounting.
-
-``void *data``
-	arbitrary mount options, usually comes as an ASCII string (see
-	"Mount Options" section)
-
-The mount() method must return the root dentry of the tree requested by
-caller.  An active reference to its superblock must be grabbed and the
-superblock must be locked.  On failure it should return ERR_PTR(error).
-
-The arguments match those of mount(2) and their interpretation depends
-on filesystem type.  E.g. for block filesystems, dev_name is interpreted
-as block device name, that device is opened and if it contains a
-suitable filesystem image the method creates and initializes struct
-super_block accordingly, returning its root dentry to caller.
-
-->mount() may choose to return a subtree of existing filesystem - it
-doesn't have to create a new one.  The main result from the caller's
-point of view is a reference to dentry at the root of (sub)tree to be
-attached; creation of new superblock is a common side effect.
-
-The most interesting member of the superblock structure that the mount()
-method fills in is the "s_op" field.  This is a pointer to a "struct
-super_operations" which describes the next level of the filesystem
-implementation.
-
-For more information on mounting (and the new mount API), see
-Documentation/filesystems/mount_api.rst.
-
 The Superblock Object
 =====================
 
@@ -244,7 +197,6 @@ filesystem.  The following members are defined:
 					enum freeze_wholder who);
 		int (*unfreeze_fs) (struct super_block *);
 		int (*statfs) (struct dentry *, struct kstatfs *);
-		int (*remount_fs) (struct super_block *, int *, char *);
 		void (*umount_begin) (struct super_block *);
 
 		int (*show_options)(struct seq_file *, struct dentry *);
@@ -351,10 +303,6 @@ or bottom half).
 ``statfs``
 	called when the VFS needs to get filesystem statistics.
 
-``remount_fs``
-	called when the filesystem is remounted.  This is called with
-	the kernel lock held
-
 ``umount_begin``
 	called when the VFS is unmounting a filesystem.