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, 2 insertions, 4 deletions

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index e23a86cae5a..f62d45e8761 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -82,7 +82,6 @@ enum rq_cmd_type_bits {
 enum {
         REQ_LB_OP_EJECT = 0x40,         /* eject request */
         REQ_LB_OP_FLUSH = 0x41,         /* flush request */
-        REQ_LB_OP_DISCARD = 0x42,       /* discard sectors */
 };
 
 /*
@@ -261,7 +260,6 @@ typedef void (request_fn_proc) (struct request_queue *q);
 typedef int (make_request_fn) (struct request_queue *q, struct bio *bio);
 typedef int (prep_rq_fn) (struct request_queue *, struct request *);
 typedef void (unplug_fn) (struct request_queue *);
-typedef int (prepare_discard_fn) (struct request_queue *, struct request *);
 
 struct bio_vec;
 struct bvec_merge_data {
@@ -340,7 +338,6 @@ struct request_queue
         make_request_fn         *make_request_fn;
         prep_rq_fn              *prep_rq_fn;
         unplug_fn               *unplug_fn;
-        prepare_discard_fn      *prepare_discard_fn;
         merge_bvec_fn           *merge_bvec_fn;
         prepare_flush_fn        *prepare_flush_fn;
         softirq_done_fn         *softirq_done_fn;
@@ -460,6 +457,7 @@ struct request_queue
 #define QUEUE_FLAG_VIRT        QUEUE_FLAG_NONROT /* paravirt device */
 #define QUEUE_FLAG_IO_STAT     15       /* do IO stats */
 #define QUEUE_FLAG_CQ          16       /* hardware does queuing */
+#define QUEUE_FLAG_DISCARD     17       /* supports DISCARD */
 
 #define QUEUE_FLAG_DEFAULT      ((1 << QUEUE_FLAG_IO_STAT) |            \
                                  (1 << QUEUE_FLAG_CLUSTER) |            \
@@ -591,6 +589,7 @@ enum {
 #define blk_queue_flushing(q)   ((q)->ordseq)
 #define blk_queue_stackable(q)  \
         test_bit(QUEUE_FLAG_STACKABLE, &(q)->queue_flags)
+#define blk_queue_discard(q)    test_bit(QUEUE_FLAG_DISCARD, &(q)->queue_flags)
 
 #define blk_fs_request(rq)      ((rq)->cmd_type == REQ_TYPE_FS)
 #define blk_pc_request(rq)      ((rq)->cmd_type == REQ_TYPE_BLOCK_PC)
@@ -955,7 +954,6 @@ extern void blk_queue_merge_bvec(struct request_queue *, merge_bvec_fn *);
 extern void blk_queue_dma_alignment(struct request_queue *, int);
 extern void blk_queue_update_dma_alignment(struct request_queue *, int);
 extern void blk_queue_softirq_done(struct request_queue *, softirq_done_fn *);
-extern void blk_queue_set_discard(struct request_queue *, prepare_discard_fn *);
 extern void blk_queue_rq_timed_out(struct request_queue *, rq_timed_out_fn *);
 extern void blk_queue_rq_timeout(struct request_queue *, unsigned int);
 extern struct backing_dev_info *blk_get_backing_dev_info(struct block_device *bdev);