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, 1 deletions

diff --git a/drivers/staging/dst/dcore.c b/drivers/staging/dst/dcore.c
index ac8577358ba0..5e8db0677582 100644
--- a/drivers/staging/dst/dcore.c
+++ b/drivers/staging/dst/dcore.c
@@ -102,7 +102,7 @@ static int dst_request(struct request_queue *q, struct bio *bio)
         struct dst_node *n = q->queuedata;
         int err = -EIO;
 
-        if (bio_empty_barrier(bio) && !q->prepare_discard_fn) {
+        if (bio_empty_barrier(bio) && !blk_queue_discard(q)) {
                 /*
                  * This is a dirty^Wnice hack, but if we complete this
                  * operation with -EOPNOTSUPP like intended, XFS