splice: Make splice from a DAX file use copy_splice_read()

Make a read splice from a DAX file go directly to copy_splice_read() to do
the reading as filemap_splice_read() is unlikely to find any pagecache to
splice.

I think this affects only erofs, Ext2, Ext4, fuse and XFS.

Signed-off-by: David Howells <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Christian Brauner <[email protected]>
Reviewed-by: Theodore Ts'o <[email protected]>
Reviewed-by: Gao Xiang <[email protected]>
cc: Al Viro <[email protected]>
cc: Jens Axboe <[email protected]>
cc: [email protected]
cc: [email protected]
cc: [email protected]
cc: [email protected]
cc: [email protected]
cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: dhowells

fs/splice.c

@@ -908,10 +908,10 @@ long vfs_splice_read(struct file *in, loff_t *ppos,
 	if (unlikely(!in->f_op->splice_read))
 		return warn_unsupported(in, "read");
 	/*
-	 * O_DIRECT doesn't deal with the pagecache, so we allocate a buffer,
-	 * copy into it and splice that into the pipe.
+	 * O_DIRECT and DAX don't deal with the pagecache, so we allocate a
+	 * buffer, copy into it and splice that into the pipe.
 	 */
-	if ((in->f_flags & O_DIRECT))
+	if ((in->f_flags & O_DIRECT) || IS_DAX(in->f_mapping->host))
 		return copy_splice_read(in, ppos, pipe, len, flags);
 	return in->f_op->splice_read(in, ppos, pipe, len, flags);
 }