iomap: Optimize iomap_read_folio

iomap_readpage_iter() handles "uptodate blocks" and "not uptodate blocks"
within a folio separately. This makes iomap_read_folio() to call into
->iomap_begin() to request for extent mapping even though it might already
have an extent which is not fully processed.
This happens when we either have a large folio or with bs < ps. In these
cases we can have sub blocks which can be uptodate (say for e.g. due to
previous writes). With iomap_read_folio_iter(), this is handled more
efficiently by not calling ->iomap_begin() call until all the sub blocks
with the current folio are processed.

iomap_read_folio_iter() handles multiple sub blocks within a given
folio but it's implementation logic is similar to how
iomap_readahead_iter() handles multiple folios within a single mapped
extent. Both of them iterate over a given range of folio/mapped extent
and call iomap_readpage_iter() for reading.
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Link: https://lore.kernel.org/r/92ae9f3333c9a7e66214568d08f45664261c899c.1715067055.git.ritesh.list@gmail.comReviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
cc: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/iomap/buffered-io.c

@@ -444,6 +444,24 @@ static loff_t iomap_readpage_iter(const struct iomap_iter *iter,
 	return pos - orig_pos + plen;
 }
 
+static loff_t iomap_read_folio_iter(const struct iomap_iter *iter,
+		struct iomap_readpage_ctx *ctx)
+{
+	struct folio *folio = ctx->cur_folio;
+	size_t offset = offset_in_folio(folio, iter->pos);
+	loff_t length = min_t(loff_t, folio_size(folio) - offset,
+			      iomap_length(iter));
+	loff_t done, ret;
+
+	for (done = 0; done < length; done += ret) {
+		ret = iomap_readpage_iter(iter, ctx, done);
+		if (ret <= 0)
+			return ret;
+	}
+
+	return done;
+}
+
 int iomap_read_folio(struct folio *folio, const struct iomap_ops *ops)
 {
 	struct iomap_iter iter = {
@@ -459,7 +477,7 @@ int iomap_read_folio(struct folio *folio, const struct iomap_ops *ops)
 	trace_iomap_readpage(iter.inode, 1);
 
 	while ((ret = iomap_iter(&iter, ops)) > 0)
-		iter.processed = iomap_readpage_iter(&iter, &ctx, 0);
+		iter.processed = iomap_read_folio_iter(&iter, &ctx);
 
 	if (ret < 0)
 		folio_set_error(folio);