linux-ext4 - [PATCHv2 2/2] iomap: Optimize iomap_read

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Message-ID: <92ae9f3333c9a7e66214568d08f45664261c899c.1715067055.git.ritesh.list@gmail.com>
Date: Tue,  7 May 2024 14:25:43 +0530
From: "Ritesh Harjani (IBM)" <ritesh.list@...il.com>
To: linux-xfs@...r.kernel.org
Cc: linux-ext4@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	Matthew Wilcox <willy@...radead.org>,
	"Darrick J . Wong" <djwong@...nel.org>,
	Ojaswin Mujoo <ojaswin@...ux.ibm.com>,
	Ritesh Harjani <ritesh.list@...il.com>,
	Jan Kara <jack@...e.cz>
Subject: [PATCHv2 2/2] 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@...il.com>
cc: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
---
 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..a9bd74ee7870 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 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);
--
2.44.0