block: improve handling of the magic discard payload
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>
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 47a5c87..9a8ccff 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -1007,8 +1007,8 @@ static int scsi_init_sgtable(struct request *req, struct scsi_data_buffer *sdb)
/*
* If sg table allocation fails, requeue request later.
*/
- if (unlikely(sg_alloc_table_chained(&sdb->table, req->nr_phys_segments,
- sdb->table.sgl)))
+ if (unlikely(sg_alloc_table_chained(&sdb->table,
+ blk_rq_nr_phys_segments(req), sdb->table.sgl)))
return BLKPREP_DEFER;
/*
@@ -1040,7 +1040,7 @@ int scsi_init_io(struct scsi_cmnd *cmd)
bool is_mq = (rq->mq_ctx != NULL);
int error;
- BUG_ON(!rq->nr_phys_segments);
+ BUG_ON(!blk_rq_nr_phys_segments(rq));
error = scsi_init_sgtable(rq, &cmd->sdb);
if (error)