block: bio: ensure newly added bio flags don't override BVEC_POOL_IDX

With the introduction of BIO_NO_PAGE_REF we've used up all available bits
in bio::bi_flags.

Convert the defines of the flags to an enum and add a BUILD_BUG_ON() call
to make sure no-one adds a new one and thus overrides the BVEC_POOL_IDX
causing crashes.
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bio.c

@@ -2218,6 +2218,9 @@ static int __init init_bio(void)
 	bio_slab_nr = 0;
 	bio_slabs = kcalloc(bio_slab_max, sizeof(struct bio_slab),
 			    GFP_KERNEL);
+
+	BUILD_BUG_ON(BIO_FLAG_LAST > BVEC_POOL_OFFSET);
+
 	if (!bio_slabs)
 		panic("bio: can't allocate bios\n");
 

include/linux/blk_types.h

@@ -215,21 +215,24 @@ struct bio {
 /*
  * bio flags
  */
-#define BIO_NO_PAGE_REF	0	/* don't put release vec pages */
-#define BIO_SEG_VALID	1	/* bi_phys_segments valid */
-#define BIO_CLONED	2	/* doesn't own data */
-#define BIO_BOUNCED	3	/* bio is a bounce bio */
-#define BIO_USER_MAPPED 4	/* contains user pages */
-#define BIO_NULL_MAPPED 5	/* contains invalid user pages */
-#define BIO_QUIET	6	/* Make BIO Quiet */
-#define BIO_CHAIN	7	/* chained bio, ->bi_remaining in effect */
-#define BIO_REFFED	8	/* bio has elevated ->bi_cnt */
-#define BIO_THROTTLED	9	/* This bio has already been subjected to
+enum {
+	BIO_NO_PAGE_REF,	/* don't put release vec pages */
+	BIO_SEG_VALID,		/* bi_phys_segments valid */
+	BIO_CLONED,		/* doesn't own data */
+	BIO_BOUNCED,		/* bio is a bounce bio */
+	BIO_USER_MAPPED,	/* contains user pages */
+	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. */
-#define BIO_TRACE_COMPLETION 10	/* bio_endio() should trace the final completion
+	BIO_TRACE_COMPLETION,	/* bio_endio() should trace the final completion
 				 * of this bio. */
-#define BIO_QUEUE_ENTERED 11	/* can use blk_queue_enter_live() */
-#define BIO_TRACKED 12		/* set if bio goes through the rq_qos path */
+	BIO_QUEUE_ENTERED,	/* can use blk_queue_enter_live() */
+	BIO_TRACKED,		/* set if bio goes through the rq_qos path */
+	BIO_FLAG_LAST
+};
 
 /* See BVEC_POOL_OFFSET below before adding new flags */