[RFCv3 6/7] iomap: Optimize iomap_read_folio

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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.

Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx>
cc: Ojaswin Mujoo <ojaswin@xxxxxxxxxxxxx>
---
 fs/iomap/buffered-io.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 9f79c82d1f73..0a4269095ae2 100644
--- a/fs/iomap/buffered-io.c
+++ b/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 pos = offset_in_folio(folio, iter->pos);
+	loff_t length = min_t(loff_t, folio_size(folio) - pos,
+			      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);
-- 
2.44.0





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