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block: improve handling of the magic discard payload

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Instead of allocating a single unused biovec for discard requests, send
them down without any payload.  Instead we allow the driver to add a
"special" payload using a biovec embedded into struct request (unioned
over other fields never used while in the driver), and overloading
the number of segments for this case.

This has a couple of advantages:

 - we don't have to allocate the bio_vec
 - the amount of special casing for discard requests in the block
   layer is significantly reduced
 - using this same scheme for other request types is trivial,
   which will be important for implementing the new WRITE_ZEROES
   op on devices where it actually requires a payload (e.g. SCSI)
 - we can get rid of playing games with the request length, as
   we'll never touch it and completions will work just fine
 - it will allow us to support ranged discard operations in the
   future by merging non-contiguous discard bios into a single
   request
 - last but not least it removes a lot of code

This patch is the common base for my WIP series for ranges discards and to
remove discard_zeroes_data in favor of always using REQ_OP_WRITE_ZEROES,
so it would be good to get it in quickly.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>
This commit is contained in:
Christoph Hellwig 2016-12-08 15:20:32 -07:00 • committed by Jens Axboe
parent be07e14f96
commit f9d03f96b9
13 changed files with 76 additions and 138 deletions
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15
include/linux/blkdev.h
View file

@ -120,10 +120,13 @@ typedef __u32 __bitwise req_flags_t;
#define RQF_HASHED ((__force req_flags_t)(1 << 16))
/* IO stats tracking on */
#define RQF_STATS ((__force req_flags_t)(1 << 17))
/* Look at ->special_vec for the actual data payload instead of the
bio chain. */
#define RQF_SPECIAL_PAYLOAD ((__force req_flags_t)(1 << 18))
/* flags that prevent us from merging requests: */
#define RQF_NOMERGE_FLAGS \
(RQF_STARTED | RQF_SOFTBARRIER | RQF_FLUSH_SEQ)
(RQF_STARTED | RQF_SOFTBARRIER | RQF_FLUSH_SEQ | RQF_SPECIAL_PAYLOAD)
#define BLK_MAX_CDB 16
@ -175,6 +178,7 @@ struct request {
*/
union {
struct rb_node rb_node; /* sort/lookup */
struct bio_vec special_vec;
void *completion_data;
};
@ -909,8 +913,6 @@ extern void __blk_put_request(struct request_queue *, struct request *);
extern struct request *blk_get_request(struct request_queue *, int, gfp_t);
extern void blk_rq_set_block_pc(struct request *);
extern void blk_requeue_request(struct request_queue *, struct request *);
extern void blk_add_request_payload(struct request *rq, struct page *page,
int offset, unsigned int len);
extern int blk_lld_busy(struct request_queue *q);
extern int blk_rq_prep_clone(struct request *rq, struct request *rq_src,
struct bio_set *bs, gfp_t gfp_mask,
@ -1153,6 +1155,13 @@ extern void blk_queue_flush_queueable(struct request_queue *q, bool queueable);
extern void blk_queue_write_cache(struct request_queue *q, bool enabled, bool fua);
extern struct backing_dev_info *blk_get_backing_dev_info(struct block_device *bdev);
static inline unsigned short blk_rq_nr_phys_segments(struct request *rq)
{
if (rq->rq_flags & RQF_SPECIAL_PAYLOAD)
return 1;
return rq->nr_phys_segments;
}
extern int blk_rq_map_sg(struct request_queue *, struct request *, struct scatterlist *);
extern void blk_dump_rq_flags(struct request *, char *);
extern long nr_blockdev_pages(void);