Re: [PATCH] block: preserve BIO_REFFED flag in bio_reset()

[Christoph Hellwig <hch@infradead.org>]

On Sun, Apr 07, 2019 at 05:35:45PM +0300, Alex Lyakas wrote:
> Hi Christoph,
> 
> I understand and agree with your concern.
> 
> Looking at the code, bi_flags is 16-bits wide, and only top 3 bits are
> preserved by bio_reset(). These bits are used to indicate a bvec pool.
> So we don't have any free bits to move the BIO_REFFED to. Unless we
> make bi_flags wider.

I think we just need to move BIO_REFFED around.  Something like the
untested patch below should do the job, although blk_types.h could
use some additional cleanup in this area..

diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index be418275763c..017a450dc8e0 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -224,13 +224,13 @@ enum {
 	BIO_NULL_MAPPED,	/* contains invalid user pages */
 	BIO_QUIET,		/* Make BIO Quiet */
 	BIO_CHAIN,		/* chained bio, ->bi_remaining in effect */
-	BIO_REFFED,		/* bio has elevated ->bi_cnt */
 	BIO_THROTTLED,		/* This bio has already been subjected to
 				 * throttling rules. Don't do it again. */
 	BIO_TRACE_COMPLETION,	/* bio_endio() should trace the final completion
 				 * of this bio. */
 	BIO_QUEUE_ENTERED,	/* can use blk_queue_enter_live() */
 	BIO_TRACKED,		/* set if bio goes through the rq_qos path */
+	BIO_REFFED,		/* bio has elevated ->bi_cnt */
 	BIO_FLAG_LAST
 };
 
@@ -257,9 +257,9 @@ enum {
 
 /*
  * Flags starting here get preserved by bio_reset() - this includes
- * only BVEC_POOL_IDX()
+ * BVEC_POOL_IDX()
  */
-#define BIO_RESET_BITS	BVEC_POOL_OFFSET
+#define BIO_RESET_BITS BIO_REFFED
 
 typedef __u32 __bitwise blk_mq_req_flags_t;