[RFCv2 2/8] libfs: Add __generic_file_fsync_nolock implementation

["Ritesh Harjani (IBM)" <ritesh.list@gmail.com>]

Some of the higher layers like iomap takes inode_lock() when calling
generic_write_sync().
Also writeback already happens from other paths without inode lock,
so it's difficult to say that we really need sync_mapping_buffers() to
take any inode locking here. Having said that, let's add a _nolock
variant of this function in libfs for now so that filesystems like
ext2 and ext4's nojournal mode can use it.

Ext4 when got converted to iomap for direct-io already copied it's own
variant of __generic_file_fsync() without lock. Hence let's add a helper
API and use it both in ext2 and ext4.

Later we can review other filesystems as well to see if we can make
_nolock as the default path if inode_lock() is not necessary here.

Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
---
 fs/libfs.c         | 43 +++++++++++++++++++++++++++++++++++++++++++
 include/linux/fs.h |  1 +
 2 files changed, 44 insertions(+)

diff --git a/fs/libfs.c b/fs/libfs.c
index 4eda519c3002..d2dfb72e3cf8 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -1110,6 +1110,49 @@ struct dentry *generic_fh_to_parent(struct super_block *sb, struct fid *fid,
 }
 EXPORT_SYMBOL_GPL(generic_fh_to_parent);
 
+/**
+ * __generic_file_fsync_nolock - generic fsync implementation for simple
+ * filesystems with no inode lock
+ *
+ * @file:	file to synchronize
+ * @start:	start offset in bytes
+ * @end:	end offset in bytes (inclusive)
+ * @datasync:	only synchronize essential metadata if true
+ *
+ * This is a generic implementation of the fsync method for simple
+ * filesystems which track all non-inode metadata in the buffers list
+ * hanging off the address_space structure.
+ */
+int __generic_file_fsync_nolock(struct file *file, loff_t start, loff_t end,
+				 int datasync)
+{
+	struct inode *inode = file->f_mapping->host;
+	int err;
+	int ret;
+
+	err = file_write_and_wait_range(file, start, end);
+	if (err)
+		return err;
+
+	ret = sync_mapping_buffers(inode->i_mapping);
+	if (!(inode->i_state & I_DIRTY_ALL))
+		goto out;
+	if (datasync && !(inode->i_state & I_DIRTY_DATASYNC))
+		goto out;
+
+	err = sync_inode_metadata(inode, 1);
+	if (ret == 0)
+		ret = err;
+
+out:
+	/* check and advance again to catch errors after syncing out buffers */
+	err = file_check_and_advance_wb_err(file);
+	if (ret == 0)
+		ret = err;
+	return ret;
+}
+EXPORT_SYMBOL(__generic_file_fsync_nolock);
+
 /**
  * __generic_file_fsync - generic fsync implementation for simple filesystems
  *
diff --git a/include/linux/fs.h b/include/linux/fs.h
index c85916e9f7db..21d2b5670308 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2935,6 +2935,7 @@ extern ssize_t simple_read_from_buffer(void __user *to, size_t count,
 extern ssize_t simple_write_to_buffer(void *to, size_t available, loff_t *ppos,
 		const void __user *from, size_t count);
 
+extern int __generic_file_fsync_nolock(struct file *, loff_t, loff_t, int);
 extern int __generic_file_fsync(struct file *, loff_t, loff_t, int);
 extern int generic_file_fsync(struct file *, loff_t, loff_t, int);
 
-- 
2.39.2