Re: [PATCH v2 03/15] block: Support data lifetime in the I/O priority bitfield

[Bart Van Assche <bvanassche@acm.org>]

On 10/11/23 18:02, Damien Le Moal wrote:
> Some have stated interest in CDL in NVMe-oF context, which could
> imply that combining CDL and lifetime may be something useful to do
> in that space...

We are having this discussion because bi_ioprio is sixteen bits wide and
because we don't want to make struct bio larger. How about expanding the
bi_ioprio field from 16 to 32 bits and to use separate bits for CDL
information and data lifetimes?

This patch does not make struct bio bigger because it changes a three
byte hole with a one byte hole:

--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -268,8 +268,8 @@ struct bio {
                                                  * req_flags.
                                                  */
         unsigned short          bi_flags;       /* BIO_* below */
-       unsigned short          bi_ioprio;
         blk_status_t            bi_status;
+       u32                     bi_ioprio;
         atomic_t                __bi_remaining;

         struct bvec_iter        bi_iter;


 From the pahole output:

struct bio {
         struct bio *               bi_next;              /*     0     8 */
         struct block_device *      bi_bdev;              /*     8     8 */
         blk_opf_t                  bi_opf;               /*    16     4 */
         short unsigned int         bi_flags;             /*    20     2 */
         blk_status_t               bi_status;            /*    22     1 */

         /* XXX 1 byte hole, try to pack */

         u32                        bi_ioprio;            /*    24     4 */
         atomic_t                   __bi_remaining;       /*    28     4 */
         struct bvec_iter           bi_iter;              /*    32    20 */
         blk_qc_t                   bi_cookie;            /*    52     4 */
         bio_end_io_t *             bi_end_io;            /*    56     8 */
         /* --- cacheline 1 boundary (64 bytes) --- */
         void *                     bi_private;           /*    64     8 */
         struct bio_crypt_ctx *     bi_crypt_context;     /*    72     8 */
         union {
                 struct bio_integrity_payload * bi_integrity; /*    80 
   8 */
         };                                               /*    80     8 */
         short unsigned int         bi_vcnt;              /*    88     2 */
         short unsigned int         bi_max_vecs;          /*    90     2 */
         atomic_t                   __bi_cnt;             /*    92     4 */
         struct bio_vec *           bi_io_vec;            /*    96     8 */
         struct bio_set *           bi_pool;              /*   104     8 */
         struct bio_vec             bi_inline_vecs[];     /*   112     0 */

         /* size: 112, cachelines: 2, members: 19 */
         /* sum members: 111, holes: 1, sum holes: 1 */
         /* last cacheline: 48 bytes */
};

Thanks,

Bart.