[PATCH RFC] erofs: file-backed mount supports direct io

[Hongbo Li]

erofs has add file-backed mount support. In this scenario, only buffer
io is allowed. So we enhance the io mode by implementing the direct
io. Also, this can make the iov_iter (user buffer) interact with the
backed file's page cache directly.

Signed-off-by: Hongbo Li <lihongbo22 at huawei.com>
---
 fs/erofs/data.c   |  11 +++--
 fs/erofs/fileio.c | 122 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 130 insertions(+), 3 deletions(-)

diff --git a/fs/erofs/data.c b/fs/erofs/data.c
index 0cd6b5c4df98..b5baff61be16 100644
--- a/fs/erofs/data.c
+++ b/fs/erofs/data.c
@@ -395,9 +395,14 @@ static ssize_t erofs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
 	if (IS_DAX(inode))
 		return dax_iomap_rw(iocb, to, &erofs_iomap_ops);
 #endif
-	if ((iocb->ki_flags & IOCB_DIRECT) && inode->i_sb->s_bdev)
-		return iomap_dio_rw(iocb, to, &erofs_iomap_ops,
-				    NULL, 0, NULL, 0);
+	if (iocb->ki_flags & IOCB_DIRECT) {
+		if (inode->i_sb->s_bdev)
+			return iomap_dio_rw(iocb, to, &erofs_iomap_ops,
+						NULL, 0, NULL, 0);
+		if (erofs_is_fileio_mode(EROFS_SB(inode->i_sb)))
+			return generic_file_read_iter(iocb, to);
+	}
+
 	return filemap_read(iocb, to, 0);
 }
 
diff --git a/fs/erofs/fileio.c b/fs/erofs/fileio.c
index 33f8539dda4a..76ed16a8ee75 100644
--- a/fs/erofs/fileio.c
+++ b/fs/erofs/fileio.c
@@ -10,12 +10,17 @@ struct erofs_fileio_rq {
 	struct bio bio;
 	struct kiocb iocb;
 	struct super_block *sb;
+	ssize_t ret;
+	void *private;
 };
 
 struct erofs_fileio {
+	struct file *file;
 	struct erofs_map_blocks map;
 	struct erofs_map_dev dev;
 	struct erofs_fileio_rq *rq;
+	size_t total;
+	size_t done;
 };
 
 static void erofs_fileio_ki_complete(struct kiocb *iocb, long ret)
@@ -24,6 +29,7 @@ static void erofs_fileio_ki_complete(struct kiocb *iocb, long ret)
 			container_of(iocb, struct erofs_fileio_rq, iocb);
 	struct folio_iter fi;
 
+	rq->ret = ret;
 	if (ret > 0) {
 		if (ret != rq->bio.bi_iter.bi_size) {
 			bio_advance(&rq->bio, ret);
@@ -43,6 +49,17 @@ static void erofs_fileio_ki_complete(struct kiocb *iocb, long ret)
 	kfree(rq);
 }
 
+static void erofs_fileio_end_io(struct bio *bio)
+{
+	struct erofs_fileio_rq *rq =
+			container_of(bio, struct erofs_fileio_rq, bio);
+	struct erofs_fileio *io = rq->private;
+
+	if (rq->ret > 0) {
+		io->done += rq->ret;
+	}
+}
+
 static void erofs_fileio_rq_submit(struct erofs_fileio_rq *rq)
 {
 	struct iov_iter iter;
@@ -189,7 +206,112 @@ static void erofs_fileio_readahead(struct readahead_control *rac)
 	erofs_fileio_rq_submit(io.rq);
 }
 
+static int erofs_fileio_scan_iter(struct erofs_fileio *io, struct kiocb *iocb,
+				  struct iov_iter *iter)
+{
+	struct file *file = iocb->ki_filp;
+	struct inode *inode = file_inode(file);
+	struct erofs_map_blocks *map = &io->map;
+	struct iov_iter dest_iter = *iter;
+	unsigned int cur = 0, end = io->total, len;
+	loff_t pos = iocb->ki_pos;
+	int err = 0;
+
+	while (cur < end) {
+		/* submit the last fileio rq */
+		if (io->rq) {
+			erofs_fileio_rq_submit(io->rq);
+			io->rq = NULL;
+		}
+
+		if (!in_range(pos + cur, map->m_la, map->m_llen)) {
+			map->m_la = pos + cur;
+			map->m_llen = end - cur;
+			err = erofs_map_blocks(inode, map);
+			if (err)
+				break;
+		}
+
+		len = min_t(loff_t, map->m_llen, end - cur);
+		/* split the whole iter with (cur, len) */
+		dest_iter = *iter;
+		iov_iter_advance(&dest_iter, cur);
+		iov_iter_truncate(&dest_iter, len);
+		if (map->m_flags & EROFS_MAP_META) {
+			struct erofs_buf buf = __EROFS_BUF_INITIALIZER;
+			void *src;
+
+			src = erofs_read_metabuf(&buf, inode->i_sb, map->m_pa, EROFS_KMAP);
+			if (IS_ERR(src)) {
+				err = PTR_ERR(src);
+				break;
+			}
+			if (copy_to_iter(src, len, &dest_iter) != len) {
+				erofs_put_metabuf(&buf);
+				err = -EIO;
+				break;
+			}
+			erofs_put_metabuf(&buf);
+			io->done += len;
+		} else if (!(map->m_flags & EROFS_MAP_MAPPED)) {
+			iov_iter_zero(len, &dest_iter);
+			io->done += len;
+		} else {
+			io->dev = (struct erofs_map_dev) {
+				.m_pa = map->m_pa,
+				.m_deviceid = map->m_deviceid,
+			};
+			err = erofs_map_dev(inode->i_sb, &io->dev);
+			if (err)
+				break;
+			io->rq = erofs_fileio_rq_alloc(&io->dev);
+			io->rq->private = io;
+			io->rq->bio.bi_iter.bi_sector = io->dev.m_pa >> SECTOR_SHIFT;
+			io->rq->bio.bi_end_io = erofs_fileio_end_io;
+
+			if (bio_iov_iter_get_pages(&io->rq->bio, &dest_iter)) {
+				err = -EIO;
+				break;
+			}
+			io->dev.m_pa += len;
+		}
+		cur += len;
+	}
+
+	return err;
+}
+
+static ssize_t erofs_fileio_direct_io(struct kiocb *iocb, struct iov_iter *iter)
+{
+	struct file *file = iocb->ki_filp;
+	struct inode *inode = file_inode(file);
+	size_t i_size = i_size_read(inode);
+	struct erofs_fileio io = {};
+	int err = 0;
+
+	if (unlikely(iocb->ki_pos >= i_size))
+		return 0;
+
+	iter->count = min_t(size_t, iter->count,
+			    max_t(size_t, 0, i_size - iocb->ki_pos));
+
+	io.total = iter->count;
+	if (!io.total)
+		return 0;
+
+	err = erofs_fileio_scan_iter(&io, iocb, iter);
+	if (err)
+		return err;
+	erofs_fileio_rq_submit(io.rq);
+	if (io.total != io.done)
+		return -EIO;
+
+	iov_iter_advance(iter, io.done);
+	return io.done;
+}
+
 const struct address_space_operations erofs_fileio_aops = {
 	.read_folio = erofs_fileio_read_folio,
 	.readahead = erofs_fileio_readahead,
+	.direct_IO = erofs_fileio_direct_io,
 };
-- 
2.34.1