[PATCH 13/14] block: Allow REQ_FUA|REQ_READ

[Kent Overstreet <kent.overstreet@linux.dev>]

REQ_FUA|REQ_READ means "do a read that bypasses the controller cache",
the same as writes.

This is useful for when the filesystem gets a checksum error, it's
possible that a bit was flipped in the controller cache, and when we
retry we want to retry the entire IO, not just from cache.

The nvme driver already passes through REQ_FUA for reads, not just
writes, so disabling the warning is sufficient to start using it, and
bcachefs is implementing additional retries for checksum errors so can
immediately use it.

Link: https://lore.kernel.org/linux-block/20250311133517.3095878-1-kent.overstreet@linux.dev/
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
---
 block/blk-core.c | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index d6c4fa3943b5..7b1103eb877d 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -793,20 +793,21 @@ void submit_bio_noacct(struct bio *bio)
 			goto end_io;
 	}
 
+	if (WARN_ON_ONCE((bio->bi_opf & REQ_PREFLUSH) &&
+			 bio_op(bio) != REQ_OP_WRITE &&
+			 bio_op(bio) != REQ_OP_ZONE_APPEND))
+		goto end_io;
+
 	/*
 	 * Filter flush bio's early so that bio based drivers without flush
 	 * support don't have to worry about them.
 	 */
-	if (op_is_flush(bio->bi_opf)) {
-		if (WARN_ON_ONCE(bio_op(bio) != REQ_OP_WRITE &&
-				 bio_op(bio) != REQ_OP_ZONE_APPEND))
+	if (op_is_flush(bio->bi_opf) &&
+	    !bdev_write_cache(bdev)) {
+		bio->bi_opf &= ~(REQ_PREFLUSH | REQ_FUA);
+		if (!bio_sectors(bio)) {
+			status = BLK_STS_OK;
 			goto end_io;
-		if (!bdev_write_cache(bdev)) {
-			bio->bi_opf &= ~(REQ_PREFLUSH | REQ_FUA);
-			if (!bio_sectors(bio)) {
-				status = BLK_STS_OK;
-				goto end_io;
-			}
 		}
 	}
 
-- 
2.47.2