fs: add a ->sync_lazytime method

Allow the file system to explicitly implement lazytime syncing instead
of pigging back on generic inode dirtying.  This allows to simplify
the XFS implementation and prepares for non-blocking lazytime timestamp
updates.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20260108141934.2052404-8-hch@lst.de
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Documentation/filesystems/locking.rst

@@ -82,6 +82,7 @@ prototypes::
 	int (*fiemap)(struct inode *, struct fiemap_extent_info *, u64 start, u64 len);
 	void (*update_time)(struct inode *inode, enum fs_update_time type,
 			    int flags);
+	void (*sync_lazytime)(struct inode *inode);
 	int (*atomic_open)(struct inode *, struct dentry *,
 				struct file *, unsigned open_flag,
 				umode_t create_mode);
@@ -118,6 +119,7 @@ getattr:	no
 listxattr:	no
 fiemap:		no
 update_time:	no
+sync_lazytime:	no
 atomic_open:	shared (exclusive if O_CREAT is set in open flags)
 tmpfile:	no
 fileattr_get:	no or exclusive

Documentation/filesystems/vfs.rst

@@ -487,6 +487,7 @@ As of kernel 2.6.22, the following members are defined:
 		ssize_t (*listxattr) (struct dentry *, char *, size_t);
 		void (*update_time)(struct inode *inode, enum fs_update_time type,
 				    int flags);
+		void (*sync_lazytime)(struct inode *inode);
 		int (*atomic_open)(struct inode *, struct dentry *, struct file *,
 				   unsigned open_flag, umode_t create_mode);
 		int (*tmpfile) (struct mnt_idmap *, struct inode *, struct file *, umode_t);
@@ -643,6 +644,11 @@ otherwise noted.
 	an inode.  If this is not defined the VFS will update the inode
 	itself and call mark_inode_dirty_sync.
 
+``sync_lazytime``:
+	called by the writeback code to update the lazy time stamps to
+	regular time stamp updates that get syncing into the on-disk
+	inode.
+
 ``atomic_open``
 	called on the last component of an open.  Using this optional
 	method the filesystem can look up, possibly create and open the

fs/fs-writeback.c

@@ -1717,7 +1717,10 @@ bool sync_lazytime(struct inode *inode)
 		return false;
 
 	trace_writeback_lazytime(inode);
-	mark_inode_dirty_sync(inode);
+	if (inode->i_op->sync_lazytime)
+		inode->i_op->sync_lazytime(inode);
+	else
+		mark_inode_dirty_sync(inode);
 	return true;
 }
 
@@ -2569,6 +2572,8 @@ void __mark_inode_dirty(struct inode *inode, int flags)
 	trace_writeback_mark_inode_dirty(inode, flags);
 
 	if (flags & I_DIRTY_INODE) {
+		bool was_dirty_time = false;
+
 		/*
 		 * Inode timestamp update will piggback on this dirtying.
 		 * We tell ->dirty_inode callback that timestamps need to
@@ -2579,6 +2584,7 @@ void __mark_inode_dirty(struct inode *inode, int flags)
 			if (inode_state_read(inode) & I_DIRTY_TIME) {
 				inode_state_clear(inode, I_DIRTY_TIME);
 				flags |= I_DIRTY_TIME;
+				was_dirty_time = true;
 			}
 			spin_unlock(&inode->i_lock);
 		}
@@ -2591,9 +2597,12 @@ void __mark_inode_dirty(struct inode *inode, int flags)
 		 * for just I_DIRTY_PAGES or I_DIRTY_TIME.
 		 */
 		trace_writeback_dirty_inode_start(inode, flags);
-		if (sb->s_op->dirty_inode)
+		if (sb->s_op->dirty_inode) {
 			sb->s_op->dirty_inode(inode,
 				flags & (I_DIRTY_INODE | I_DIRTY_TIME));
+		} else if (was_dirty_time && inode->i_op->sync_lazytime) {
+			inode->i_op->sync_lazytime(inode);
+		}
 		trace_writeback_dirty_inode(inode, flags);
 
 		/* I_DIRTY_INODE supersedes I_DIRTY_TIME. */

include/linux/fs.h

@@ -2024,6 +2024,7 @@ struct inode_operations {
 		      u64 len);
 	int (*update_time)(struct inode *inode, enum fs_update_time type,
 			   unsigned int flags);
+	void (*sync_lazytime)(struct inode *inode);
 	int (*atomic_open)(struct inode *, struct dentry *,
 			   struct file *, unsigned open_flag,
 			   umode_t create_mode);