iomap: add read_folio_range() handler for buffered writes

Add a read_folio_range() handler for buffered writes that filesystems
may pass in if they wish to provide a custom handler for synchronously
reading in the contents of a folio.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
[hch: renamed to read_folio_range, pass less arguments]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/20250710133343.399917-14-hch@lst.de
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Christoph Hellwig

Documentation/filesystems/iomap/operations.rst

@@ -68,6 +68,8 @@ The following address space operations can be wrapped easily:
      void (*put_folio)(struct inode *inode, loff_t pos, unsigned copied,
                        struct folio *folio);
      bool (*iomap_valid)(struct inode *inode, const struct iomap *iomap);
+     int (*read_folio_range)(const struct iomap_iter *iter,
+     			struct folio *folio, loff_t pos, size_t len);
  };
 
 iomap calls these functions:
@@ -123,6 +125,10 @@ iomap calls these functions:
     ``->iomap_valid``, then the iomap should considered stale and the
     validation failed.
 
+  - ``read_folio_range``: Called to synchronously read in the range that will
+    be written to. If this function is not provided, iomap will default to
+    submitting a bio read request.
+
 These ``struct kiocb`` flags are significant for buffered I/O with iomap:
 
  * ``IOCB_NOWAIT``: Turns on ``IOMAP_NOWAIT``.

fs/iomap/buffered-io.c

@@ -671,7 +671,8 @@ static int iomap_read_folio_range(const struct iomap_iter *iter,
 	return submit_bio_wait(&bio);
 }
 
-static int __iomap_write_begin(const struct iomap_iter *iter, size_t len,
+static int __iomap_write_begin(const struct iomap_iter *iter,
+		const struct iomap_write_ops *write_ops, size_t len,
 		struct folio *folio)
 {
 	struct iomap_folio_state *ifs;
@@ -722,8 +723,12 @@ static int __iomap_write_begin(const struct iomap_iter *iter, size_t len,
 			if (iter->flags & IOMAP_NOWAIT)
 				return -EAGAIN;
 
-			status = iomap_read_folio_range(iter, folio,
-					block_start, plen);
+			if (write_ops && write_ops->read_folio_range)
+				status = write_ops->read_folio_range(iter,
+						folio, block_start, plen);
+			else
+				status = iomap_read_folio_range(iter,
+						folio, block_start, plen);
 			if (status)
 				return status;
 		}
@@ -839,7 +844,7 @@ static int iomap_write_begin(struct iomap_iter *iter,
 	else if (srcmap->flags & IOMAP_F_BUFFER_HEAD)
 		status = __block_write_begin_int(folio, pos, len, NULL, srcmap);
 	else
-		status = __iomap_write_begin(iter, len, folio);
+		status = __iomap_write_begin(iter, write_ops, len, folio);
 
 	if (unlikely(status))
 		goto out_unlock;

include/linux/iomap.h

@@ -166,6 +166,16 @@ struct iomap_write_ops {
 	 * locked by the iomap code.
 	 */
 	bool (*iomap_valid)(struct inode *inode, const struct iomap *iomap);
+
+	/*
+	 * Optional if the filesystem wishes to provide a custom handler for
+	 * reading in the contents of a folio, otherwise iomap will default to
+	 * submitting a bio read request.
+	 *
+	 * The read must be done synchronously.
+	 */
+	int (*read_folio_range)(const struct iomap_iter *iter,
+			struct folio *folio, loff_t pos, size_t len);
 };
 
 /*