block: shrink struct bio down to 2 cache lines again

Commit bcf2843b3f8f added ->bi_error to cleanup the error passing
for struct bio, but that ended up adding 4 bytes and a 4 byte hole
to the size of struct bio. For a clean config, that bumped it from
128 bytes, to 136 bytes, on x86-64.

The ->bi_flags member is currently an unsigned long, but it fits
easily within an int. Change it to an unsigned int, adjust the
the pool offset code, and move ->bi_error into the new hole. Then
we end up with a 128 byte bio again.

Change the bio flag set/clear to use cmpxchg to ensure we don't
lose any flags when manipulating them.
Signed-off-by: Jens Axboe <axboe@fb.com>

include/linux/bio.h

@@ -306,17 +306,17 @@ static inline void bio_cnt_set(struct bio *bio, unsigned int count)
 
 static inline bool bio_flagged(struct bio *bio, unsigned int bit)
 {
-	return (bio->bi_flags & (1UL << bit)) != 0;
+	return (bio->bi_flags & (1U << bit)) != 0;
 }
 
 static inline void bio_set_flag(struct bio *bio, unsigned int bit)
 {
-	bio->bi_flags |= (1UL << bit);
+	bio->bi_flags |= (1U << bit);
 }
 
 static inline void bio_clear_flag(struct bio *bio, unsigned int bit)
 {
-	bio->bi_flags &= ~(1UL << bit);
+	bio->bi_flags &= ~(1U << bit);
 }
 
 enum bip_flags {

include/linux/blk_types.h

@@ -46,14 +46,14 @@ struct bvec_iter {
 struct bio {
 	struct bio		*bi_next;	/* request queue link */
 	struct block_device	*bi_bdev;
-	unsigned long		bi_flags;	/* status, command, etc */
+	unsigned int		bi_flags;	/* status, command, etc */
+	int			bi_error;
 	unsigned long		bi_rw;		/* bottom bits READ/WRITE,
 						 * top bits priority
 						 */
 
 	struct bvec_iter	bi_iter;
 
-	int			bi_error;
 	/* Number of segments in this BIO after
 	 * physical address coalescing is performed.
 	 */
@@ -134,7 +134,7 @@ struct bio {
  */
 #define BIO_POOL_BITS		(4)
 #define BIO_POOL_NONE		((1UL << BIO_POOL_BITS) - 1)
-#define BIO_POOL_OFFSET		(BITS_PER_LONG - BIO_POOL_BITS)
+#define BIO_POOL_OFFSET		(32 - BIO_POOL_BITS)
 #define BIO_POOL_MASK		(1UL << BIO_POOL_OFFSET)
 #define BIO_POOL_IDX(bio)	((bio)->bi_flags >> BIO_POOL_OFFSET)