[PATCH] bcachefs: fix incorrect usage of REQ_OP_FLUSH

[Christoph Hellwig <hch@lst.de>]

REQ_OP_FLUSH is only for internal use in the blk-mq and request based
drivers. File systems and other block layer consumers must use
REQ_OP_WRITE | REQ_PREFLUSH as documented in
Documentation/block/writeback_cache_control.rst.

While REQ_OP_FLUSH appears to work for blk-mq drivers it does not
get the proper flush state machine handling, and completely fails
for any bio based drivers, including all the stacking drivers.  The
block layer will also get a check in 6.8 to reject this use case
entirely.

[Note: completely untested, but as this never got fixed since the
original bug report in November:

   https://bugzilla.kernel.org/show_bug.cgi?id=218184

and the the discussion in December:

    https://lore.kernel.org/all/20231221053016.72cqcfg46vxwohcj@moria.home.lan/T/

this seems to be best way to force it]

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 fs/bcachefs/fs-io.c      | 2 +-
 fs/bcachefs/journal_io.c | 3 ++-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/bcachefs/fs-io.c b/fs/bcachefs/fs-io.c
index b0e8144ec5500c..a8500af6c7438f 100644
--- a/fs/bcachefs/fs-io.c
+++ b/fs/bcachefs/fs-io.c
@@ -79,7 +79,7 @@ void bch2_inode_flush_nocow_writes_async(struct bch_fs *c,
 			continue;
 
 		bio = container_of(bio_alloc_bioset(ca->disk_sb.bdev, 0,
-						    REQ_OP_FLUSH,
+						    REQ_OP_WRITE | REQ_PREFLUSH,
 						    GFP_KERNEL,
 						    &c->nocow_flush_bioset),
 				   struct nocow_flush, bio);
diff --git a/fs/bcachefs/journal_io.c b/fs/bcachefs/journal_io.c
index 3eb6c3f62a811b..43c76c9ad9a316 100644
--- a/fs/bcachefs/journal_io.c
+++ b/fs/bcachefs/journal_io.c
@@ -1948,7 +1948,8 @@ CLOSURE_CALLBACK(bch2_journal_write)
 			percpu_ref_get(&ca->io_ref);
 
 			bio = ca->journal.bio;
-			bio_reset(bio, ca->disk_sb.bdev, REQ_OP_FLUSH);
+			bio_reset(bio, ca->disk_sb.bdev,
+					REQ_OP_WRITE | REQ_PREFLUSH),
 			bio->bi_end_io		= journal_write_endio;
 			bio->bi_private		= ca;
 			closure_bio_submit(bio, cl);
-- 
2.39.2