Re: [PATCH 4/6] block: clear BIO_PERCPU_CACHE flag if polling isn't supported

On Fri, Aug 13, 2021 at 02:19:11PM -0600, Jens Axboe wrote:

On 8/13/21 2:17 PM, Keith Busch wrote:

quoted

On Thu, Aug 12, 2021 at 11:41:58AM -0600, Jens Axboe wrote:

quoted

Indeed. Wonder if we should make that a small helper, as any clear of
REQ_HIPRI should clear BIO_PERCPU_CACHE as well.

diff --git a/block/blk-core.c b/block/blk-core.c
index 7e852242f4cc..d2722ecd4d9b 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c

@@ -821,11 +821,8 @@ static noinline_for_stack bool submit_bio_checks(struct bio *bio)
 		}
 	}
 
-	if (!test_bit(QUEUE_FLAG_POLL, &q->queue_flags)) {
-		/* can't support alloc cache if we turn off polling */
-		bio_clear_flag(bio, BIO_PERCPU_CACHE);
-		bio->bi_opf &= ~REQ_HIPRI;
-	}
+	if (!test_bit(QUEUE_FLAG_POLL, &q->queue_flags))
+		bio_clear_hipri(bio);

Since BIO_PERCPU_CACHE doesn't work without REQ_HIRPI, should this check
look more like this?

	if (!test_bit(QUEUE_FLAG_POLL, &q->queue_flags))
		bio->bi_opf &= ~REQ_HIPRI;
	if (!(bio->bi_opf & REQ_HIPRI))
		bio_clear_flag(bio, BIO_PERCPU_CACHE);

I realise the only BIO_PERCPU_CACHE user in this series never sets it
without REQ_HIPRI, but it looks like a problem waiting to happen if
nothing enforces this pairing: someone could set the CACHE flag on a
QUEUE_FLAG_POLL enabled queue without setting HIPRI and get the wrong
bio_put() action.

I'd rather turn that into a WARN_ON or similar. But probably better to
do that on the freeing side, honestly. That'll be the most reliable way,
but a shame to add cycles to the hot path...

Yeah, it is a coding error if that happened, so a WARN sounds okay. I
also don't like adding these kinds of checks, so please feel free to not
include it if you think the usage is clear enough.