[PATCH 1/3] fs: define a new super operation to get fsid

[Amir Goldstein <amir73il@gmail.com>]

If implemented, get_fsid() can be used as a cheaper way of getting
the f_fsid memeber of kstatfs, for callers that only care about fsid.

fanotify is going to make use of that to get btrfs fsid from inode on
every event.

Suggested-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230920110429.f4wkfuls73pd55pv@quack3/
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 Documentation/filesystems/locking.rst |  2 ++
 Documentation/filesystems/vfs.rst     |  4 ++++
 fs/statfs.c                           | 14 ++++++++++++++
 include/linux/fs.h                    |  1 +
 include/linux/statfs.h                |  2 ++
 5 files changed, 23 insertions(+)

diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst
index 7be2900806c8..a367950ee755 100644
--- a/Documentation/filesystems/locking.rst
+++ b/Documentation/filesystems/locking.rst
@@ -169,6 +169,7 @@ prototypes::
 	int (*freeze_fs) (struct super_block *);
 	int (*unfreeze_fs) (struct super_block *);
 	int (*statfs) (struct dentry *, struct kstatfs *);
+	int (*get_fsid) (struct inode *, __kernel_fsid_t *);
 	int (*remount_fs) (struct super_block *, int *, char *);
 	void (*umount_begin) (struct super_block *);
 	int (*show_options)(struct seq_file *, struct dentry *);
@@ -193,6 +194,7 @@ sync_fs:		read
 freeze_fs:		write
 unfreeze_fs:		write
 statfs:			maybe(read)	(see below)
+get_fsid:               no
 remount_fs:		write
 umount_begin:		no
 show_options:		no		(namespace_sem)
diff --git a/Documentation/filesystems/vfs.rst b/Documentation/filesystems/vfs.rst
index 99acc2e98673..f30f39f056ab 100644
--- a/Documentation/filesystems/vfs.rst
+++ b/Documentation/filesystems/vfs.rst
@@ -267,6 +267,7 @@ filesystem.  The following members are defined:
 					enum freeze_wholder who);
 		int (*unfreeze_fs) (struct super_block *);
 		int (*statfs) (struct dentry *, struct kstatfs *);
+		int (*get_fsid) (struct inode *, __kernel_fsid_t *);
 		int (*remount_fs) (struct super_block *, int *, char *);
 		void (*umount_begin) (struct super_block *);
 
@@ -374,6 +375,9 @@ or bottom half).
 ``statfs``
 	called when the VFS needs to get filesystem statistics.
 
+``get_fsid``
+	called when the VFS needs to get only the f_fsid member of statfs.
+
 ``remount_fs``
 	called when the filesystem is remounted.  This is called with
 	the kernel lock held
diff --git a/fs/statfs.c b/fs/statfs.c
index 96d1c3edf289..60a6af7356b7 100644
--- a/fs/statfs.c
+++ b/fs/statfs.c
@@ -69,11 +69,25 @@ static int statfs_by_dentry(struct dentry *dentry, struct kstatfs *buf)
 	return retval;
 }
 
+int inode_get_fsid(struct inode *inode, __kernel_fsid_t *fsid)
+{
+	if (!inode->i_sb->s_op->get_fsid)
+		return -EOPNOTSUPP;
+
+	return inode->i_sb->s_op->get_fsid(inode, fsid);
+}
+EXPORT_SYMBOL(inode_get_fsid);
+
 int vfs_get_fsid(struct dentry *dentry, __kernel_fsid_t *fsid)
 {
 	struct kstatfs st;
 	int error;
 
+	/* Avoid statfs if fs supports cheaper get_fsid() */
+	error = inode_get_fsid(d_inode(dentry), fsid);
+	if (!error)
+		return 0;
+
 	error = statfs_by_dentry(dentry, &st);
 	if (error)
 		return error;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 4a40823c3c67..30d7d3347c49 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2008,6 +2008,7 @@ struct super_operations {
 	int (*thaw_super) (struct super_block *, enum freeze_holder who);
 	int (*unfreeze_fs) (struct super_block *);
 	int (*statfs) (struct dentry *, struct kstatfs *);
+	int (*get_fsid)(struct inode *, __kernel_fsid_t *);
 	int (*remount_fs) (struct super_block *, int *, char *);
 	void (*umount_begin) (struct super_block *);
 
diff --git a/include/linux/statfs.h b/include/linux/statfs.h
index 02c862686ea3..c07f6d726e39 100644
--- a/include/linux/statfs.h
+++ b/include/linux/statfs.h
@@ -43,7 +43,9 @@ struct kstatfs {
 #define ST_RELATIME	0x1000	/* update atime relative to mtime/ctime */
 #define ST_NOSYMFOLLOW	0x2000	/* do not follow symlinks */
 
+struct inode;
 struct dentry;
+extern int inode_get_fsid(struct inode *inode, __kernel_fsid_t *fsid);
 extern int vfs_get_fsid(struct dentry *dentry, __kernel_fsid_t *fsid);
 
 static inline __kernel_fsid_t u64_to_fsid(u64 v)
-- 
2.34.1