bh_submit() takes a bio_end_io allowing users to avoid the indirect
function call through bh->b_end_io, and eventually allowing us to remove
bh->b_end_io.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Jan Kara <jack@suse.cz>
---
 fs/buffer.c                 | 31 +++++++++++++++++++++++++++----
 include/linux/buffer_head.h |  1 +
 2 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index d4e9a10d2a26..ea926963529e 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1180,15 +1180,15 @@ static void buffer_set_crypto_ctx(struct bio *bio, const struct buffer_head *bh,
 			folio_pos(bh->b_folio) + bh_offset(bh), gfp_mask);
 }
 
-static void submit_bh_wbc(blk_opf_t opf, struct buffer_head *bh,
-		enum rw_hint write_hint, struct writeback_control *wbc)
+static void __bh_submit(struct buffer_head *bh, blk_opf_t opf,
+		enum rw_hint write_hint, struct writeback_control *wbc,
+		bio_end_io_t end_bio)
 {
 	const enum req_op op = opf & REQ_OP_MASK;
 	struct bio *bio;
 
 	BUG_ON(!buffer_locked(bh));
 	BUG_ON(!buffer_mapped(bh));
-	BUG_ON(!bh->b_end_io);
 	BUG_ON(buffer_delay(bh));
 	BUG_ON(buffer_unwritten(bh));
 
@@ -1213,7 +1213,7 @@ static void submit_bh_wbc(blk_opf_t opf, struct buffer_head *bh,
 
 	bio_add_folio_nofail(bio, bh->b_folio, bh->b_size, bh_offset(bh));
 
-	bio->bi_end_io = end_bio_bh_io_sync;
+	bio->bi_end_io = end_bio;
 	bio->bi_private = bh;
 
 	/* Take care of bh's that straddle the end of the device */
@@ -1227,6 +1227,29 @@ static void submit_bh_wbc(blk_opf_t opf, struct buffer_head *bh,
 	blk_crypto_submit_bio(bio);
 }
 
+static void submit_bh_wbc(blk_opf_t opf, struct buffer_head *bh,
+			  enum rw_hint write_hint,
+			  struct writeback_control *wbc)
+{
+	BUG_ON(!bh->b_end_io);
+	__bh_submit(bh, opf, write_hint, wbc, end_bio_bh_io_sync);
+}
+
+/**
+ * bh_submit - Start I/O against a buffer head
+ * @bh: The buffer head to perform I/O on.
+ * @opf: Operation and flags for bio.
+ * @end_io: The routine to call when I/O has completed.
+ *
+ * If you need to do I/O on an individual bh (instead of allowing the
+ * page cache to do I/O on the folio that it is in), call this function.
+ */
+void bh_submit(struct buffer_head *bh, blk_opf_t opf, bio_end_io_t end_io)
+{
+	__bh_submit(bh, opf, WRITE_LIFE_NOT_SET, NULL, end_io);
+}
+EXPORT_SYMBOL(bh_submit);
+
 static struct buffer_head *__bread_slow(struct buffer_head *bh)
 {
 	lock_buffer(bh);
diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index e4939e33b4b5..d59980e4adda 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -240,6 +240,7 @@ int sync_dirty_buffer(struct buffer_head *bh);
 int __sync_dirty_buffer(struct buffer_head *bh, blk_opf_t op_flags);
 void write_dirty_buffer(struct buffer_head *bh, blk_opf_t op_flags);
 void submit_bh(blk_opf_t, struct buffer_head *);
+void bh_submit(struct buffer_head *, blk_opf_t, bio_end_io_t);
 void write_boundary_block(struct block_device *bdev,
 			sector_t bblock, unsigned blocksize);
 int bh_uptodate_or_lock(struct buffer_head *bh);
-- 
2.47.3