zonefs: Provide a splice-read wrapper

Provide a splice_read wrapper for zonefs.  This does some checks before
proceeding and locks the inode across the call to filemap_splice_read() and
a size check in case of truncation.  Splicing from direct I/O is handled by
the caller.
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Christoph Hellwig <hch@lst.de>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: Jens Axboe <axboe@kernel.dk>
cc: Darrick J. Wong <djwong@kernel.org>
cc: linux-xfs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
cc: linux-block@vger.kernel.org
cc: linux-mm@kvack.org
Acked-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20230522135018.2742245-26-dhowells@redhat.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/zonefs/file.c

@@ -752,6 +752,44 @@ static ssize_t zonefs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
 	return ret;
 }
 
+static ssize_t zonefs_file_splice_read(struct file *in, loff_t *ppos,
+				       struct pipe_inode_info *pipe,
+				       size_t len, unsigned int flags)
+{
+	struct inode *inode = file_inode(in);
+	struct zonefs_inode_info *zi = ZONEFS_I(inode);
+	struct zonefs_zone *z = zonefs_inode_zone(inode);
+	loff_t isize;
+	ssize_t ret = 0;
+
+	/* Offline zones cannot be read */
+	if (unlikely(IS_IMMUTABLE(inode) && !(inode->i_mode & 0777)))
+		return -EPERM;
+
+	if (*ppos >= z->z_capacity)
+		return 0;
+
+	inode_lock_shared(inode);
+
+	/* Limit read operations to written data */
+	mutex_lock(&zi->i_truncate_mutex);
+	isize = i_size_read(inode);
+	if (*ppos >= isize)
+		len = 0;
+	else
+		len = min_t(loff_t, len, isize - *ppos);
+	mutex_unlock(&zi->i_truncate_mutex);
+
+	if (len > 0) {
+		ret = filemap_splice_read(in, ppos, pipe, len, flags);
+		if (ret == -EIO)
+			zonefs_io_error(inode, false);
+	}
+
+	inode_unlock_shared(inode);
+	return ret;
+}
+
 /*
  * Write open accounting is done only for sequential files.
  */
@@ -896,7 +934,7 @@ const struct file_operations zonefs_file_operations = {
 	.llseek		= zonefs_file_llseek,
 	.read_iter	= zonefs_file_read_iter,
 	.write_iter	= zonefs_file_write_iter,
-	.splice_read	= generic_file_splice_read,
+	.splice_read	= zonefs_file_splice_read,
 	.splice_write	= iter_file_splice_write,
 	.iopoll		= iocb_bio_iopoll,
 };