block: use normal I/O path for discard requests

prepare_discard_fn() was being called in a place where memory allocation
was effectively impossible.  This makes it inappropriate for all but
the most trivial translations of Linux's DISCARD operation to the block
command set.  Additionally adding a payload there makes the ownership
of the bio backing unclear as it's now allocated by the device driver
and not the submitter as usual.

It is replaced with QUEUE_FLAG_DISCARD which is used to indicate whether
the queue supports discard operations or not.  blkdev_issue_discard now
allocates a one-page, sector-length payload which is the right thing
for the common ATA and SCSI implementations.

The mtd implementation of prepare_discard_fn() is replaced with simply
checking for the request being a discard.

Largely based on a previous patch from Matthew Wilcox <matthew@wil.cx>
which did the prepare_discard_fn but not the different payload allocation
yet.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 1 insertions, 2 deletions

diff --git a/block/blk-core.c b/block/blk-core.c
index 8135228e4b29..80a020dd1580 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1124,7 +1124,6 @@ void init_request_from_bio(struct request *req, struct bio *bio)
                 req->cmd_flags |= REQ_DISCARD;
                 if (bio_rw_flagged(bio, BIO_RW_BARRIER))
                         req->cmd_flags |= REQ_SOFTBARRIER;
-                req->q->prepare_discard_fn(req->q, req);
         } else if (unlikely(bio_rw_flagged(bio, BIO_RW_BARRIER)))
                 req->cmd_flags |= REQ_HARDBARRIER;
 
@@ -1470,7 +1469,7 @@ static inline void __generic_make_request(struct bio *bio)
                         goto end_io;
 
                 if (bio_rw_flagged(bio, BIO_RW_DISCARD) &&
-                    !q->prepare_discard_fn) {
+                    !blk_queue_discard(q)) {
                         err = -EOPNOTSUPP;
                         goto end_io;
                 }