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>

1 files changed, 2 insertions, 32 deletions

diff --git a/block/blk-core.c b/block/blk-core.c
index 4b7ec59..bd642a4 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1475,38 +1475,6 @@ void blk_put_request(struct request *req)
 }
 EXPORT_SYMBOL(blk_put_request);
 
-/**
- * blk_add_request_payload - add a payload to a request
- * @rq: request to update
- * @page: page backing the payload
- * @offset: offset in page
- * @len: length of the payload.
- *
- * This allows to later add a payload to an already submitted request by
- * a block driver.  The driver needs to take care of freeing the payload
- * itself.
- *
- * Note that this is a quite horrible hack and nothing but handling of
- * discard requests should ever use it.
- */
-void blk_add_request_payload(struct request *rq, struct page *page,
-		int offset, unsigned int len)
-{
-	struct bio *bio = rq->bio;
-
-	bio->bi_io_vec->bv_page = page;
-	bio->bi_io_vec->bv_offset = offset;
-	bio->bi_io_vec->bv_len = len;
-
-	bio->bi_iter.bi_size = len;
-	bio->bi_vcnt = 1;
-	bio->bi_phys_segments = 1;
-
-	rq->__data_len = rq->resid_len = len;
-	rq->nr_phys_segments = 1;
-}
-EXPORT_SYMBOL_GPL(blk_add_request_payload);
-
 bool bio_attempt_back_merge(struct request_queue *q, struct request *req,
 			    struct bio *bio)
 {
@@ -2642,6 +2610,8 @@ bool blk_update_request(struct request *req, int error, unsigned int nr_bytes)
 		return false;
 	}
 
+	WARN_ON_ONCE(req->rq_flags & RQF_SPECIAL_PAYLOAD);
+
 	req->__data_len -= total_bytes;
 
 	/* update sector only for requests with clear definition of sector */