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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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24
drivers/scsi/sd.c
View File

@@ -716,7 +716,6 @@ static int sd_setup_discard_cmnd(struct scsi_cmnd *cmd)
struct scsi_disk *sdkp = scsi_disk(rq->rq_disk);
sector_t sector = blk_rq_pos(rq);
unsigned int nr_sectors = blk_rq_sectors(rq);
unsigned int nr_bytes = blk_rq_bytes(rq);
unsigned int len;
int ret;
char *buf;
@@ -772,24 +771,19 @@ static int sd_setup_discard_cmnd(struct scsi_cmnd *cmd)
goto out;
}
rq->completion_data = page;
rq->timeout = SD_TIMEOUT;
cmd->transfersize = len;
cmd->allowed = SD_MAX_RETRIES;
/*
* Initially __data_len is set to the amount of data that needs to be
* transferred to the target. This amount depends on whether WRITE SAME
* or UNMAP is being used. After the scatterlist has been mapped by
* scsi_init_io() we set __data_len to the size of the area to be
* discarded on disk. This allows us to report completion on the full
* amount of blocks described by the request.
*/
blk_add_request_payload(rq, page, 0, len);
ret = scsi_init_io(cmd);
rq->__data_len = nr_bytes;
rq->special_vec.bv_page = page;
rq->special_vec.bv_offset = 0;
rq->special_vec.bv_len = len;
rq->rq_flags |= RQF_SPECIAL_PAYLOAD;
rq->resid_len = len;
ret = scsi_init_io(cmd);
out:
if (ret != BLKPREP_OK)
__free_page(page);
@@ -1182,8 +1176,8 @@ static void sd_uninit_command(struct scsi_cmnd *SCpnt)
{
struct request *rq = SCpnt->request;
if (req_op(rq) == REQ_OP_DISCARD)
__free_page(rq->completion_data);
if (rq->rq_flags & RQF_SPECIAL_PAYLOAD)
__free_page(rq->special_vec.bv_page);
if (SCpnt->cmnd != rq->cmd) {
mempool_free(SCpnt->cmnd, sd_cdb_pool);