[PATCH v4 04/21] iomap: add ->iomap_next()

[Joanne Koong <joannelkoong@gmail.com>]

Have one ->iomap_next() callback instead of ->iomap_begin() and
->iomap_end(). ->iomap_next() finishes the previous mapping if needed,
and produces the next mapping.

Collapsing to a single callback lets a performance-critical caller
inline its iteration loop and pass its ->iomap_next() function as a
compile-time constant, so the compiler can devirtualize that callback
into a direct call instead of an indirect call through a function
pointer.

iomap_iter() uses ->iomap_next() when the filesystem provides that
callback and otherwise falls back to the ->iomap_begin()/->iomap_end()
path, so filesystems can be converted one at a time.

Suggested-by: Christoph Hellwig <hch@lst.de>
Suggested-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
---
 fs/iomap/iter.c       | 8 ++++++--
 include/linux/iomap.h | 9 +++++++++
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/fs/iomap/iter.c b/fs/iomap/iter.c
index 66ccb87441ab..e05574602946 100644
--- a/fs/iomap/iter.c
+++ b/fs/iomap/iter.c
@@ -107,8 +107,12 @@ int iomap_iter(struct iomap_iter *iter, const struct iomap_ops *ops)
 
 	trace_iomap_iter(iter, ops, _RET_IP_);
 
-	ret = iomap_iter_next(iter, &iter->iomap, &iter->srcmap,
-			ops->iomap_begin, ops->iomap_end);
+	if (ops->iomap_next)
+		ret = ops->iomap_next(iter, &iter->iomap, &iter->srcmap);
+	else
+		ret = iomap_iter_next(iter, &iter->iomap, &iter->srcmap,
+				ops->iomap_begin, ops->iomap_end);
+
 	iter->status = 0;
 	if (ret > 0)
 		iomap_iter_done(iter);
diff --git a/include/linux/iomap.h b/include/linux/iomap.h
index 80832edc7ec2..d203d9fe0f89 100644
--- a/include/linux/iomap.h
+++ b/include/linux/iomap.h
@@ -231,9 +231,18 @@ typedef int (*iomap_iter_begin_fn)(struct inode *inode, loff_t pos,
 typedef int (*iomap_iter_end_fn)(struct inode *inode, loff_t pos, loff_t length,
 		ssize_t written, unsigned flags, struct iomap *iomap);
 
+/*
+ * Produce the next mapping (finishing the previous one if needed).
+ * Return 1 to continue iterating, 0 if the range is fully consumed, or a
+ * negative error on failure.
+ */
+typedef int (*iomap_iter_next_fn)(const struct iomap_iter *iter,
+		struct iomap *iomap, struct iomap *srcmap);
+
 struct iomap_ops {
 	iomap_iter_begin_fn iomap_begin;
 	iomap_iter_end_fn iomap_end;
+	iomap_iter_next_fn iomap_next;
 };
 
 /**
-- 
2.52.0