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>

6 files changed, 39 insertions, 43 deletions

diff --git a/block/blk-barrier.c b/block/blk-barrier.c
index 6593ab39cfe9..21f5025c3945 100644
--- a/block/blk-barrier.c
+++ b/block/blk-barrier.c
@@ -350,6 +350,7 @@ static void blkdev_discard_end_io(struct bio *bio, int err)
 
         if (bio->bi_private)
                 complete(bio->bi_private);
+        __free_page(bio_page(bio));
 
         bio_put(bio);
 }
@@ -372,26 +373,44 @@ int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
         struct request_queue *q = bdev_get_queue(bdev);
         int type = flags & DISCARD_FL_BARRIER ?
                 DISCARD_BARRIER : DISCARD_NOBARRIER;
+        struct bio *bio;
+        struct page *page;
         int ret = 0;
 
         if (!q)
                 return -ENXIO;
 
-        if (!q->prepare_discard_fn)
+        if (!blk_queue_discard(q))
                 return -EOPNOTSUPP;
 
         while (nr_sects && !ret) {
-                struct bio *bio = bio_alloc(gfp_mask, 0);
-                if (!bio)
-                        return -ENOMEM;
+                unsigned int sector_size = q->limits.logical_block_size;
 
+                bio = bio_alloc(gfp_mask, 1);
+                if (!bio)
+                        goto out;
+                bio->bi_sector = sector;
                 bio->bi_end_io = blkdev_discard_end_io;
                 bio->bi_bdev = bdev;
                 if (flags & DISCARD_FL_WAIT)
                         bio->bi_private = &wait;
 
-                bio->bi_sector = sector;
+                /*
+                 * Add a zeroed one-sector payload as that's what
+                 * our current implementations need.  If we'll ever need
+                 * more the interface will need revisiting.
+                 */
+                page = alloc_page(GFP_KERNEL | __GFP_ZERO);
+                if (!page)
+                        goto out_free_bio;
+                if (bio_add_pc_page(q, bio, page, sector_size, 0) < sector_size)
+                        goto out_free_page;
 
+                /*
+                 * And override the bio size - the way discard works we
+                 * touch many more blocks on disk than the actual payload
+                 * length.
+                 */
                 if (nr_sects > queue_max_hw_sectors(q)) {
                         bio->bi_size = queue_max_hw_sectors(q) << 9;
                         nr_sects -= queue_max_hw_sectors(q);
@@ -414,5 +433,11 @@ int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
                 bio_put(bio);
         }
         return ret;
+out_free_page:
+        __free_page(page);
+out_free_bio:
+        bio_put(bio);
+out:
+        return -ENOMEM;
 }
 EXPORT_SYMBOL(blkdev_issue_discard);
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;
                 }
diff --git a/block/blk-settings.c b/block/blk-settings.c
index eaf122ff5f16..d29498ef1eb5 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -34,23 +34,6 @@ void blk_queue_prep_rq(struct request_queue *q, prep_rq_fn *pfn)
 EXPORT_SYMBOL(blk_queue_prep_rq);
 
 /**
- * blk_queue_set_discard - set a discard_sectors function for queue
- * @q:          queue
- * @dfn:        prepare_discard function
- *
- * It's possible for a queue to register a discard callback which is used
- * to transform a discard request into the appropriate type for the
- * hardware. If none is registered, then discard requests are failed
- * with %EOPNOTSUPP.
- *
- */
-void blk_queue_set_discard(struct request_queue *q, prepare_discard_fn *dfn)
-{
-        q->prepare_discard_fn = dfn;
-}
-EXPORT_SYMBOL(blk_queue_set_discard);
-
-/**
  * blk_queue_merge_bvec - set a merge_bvec function for queue
  * @q:          queue
  * @mbfn:       merge_bvec_fn
diff --git a/drivers/mtd/mtd_blkdevs.c b/drivers/mtd/mtd_blkdevs.c
index 0acbf4f5be50..8ca17a3e96ea 100644
--- a/drivers/mtd/mtd_blkdevs.c
+++ b/drivers/mtd/mtd_blkdevs.c
@@ -32,14 +32,6 @@ struct mtd_blkcore_priv {
         spinlock_t queue_lock;
 };
 
-static int blktrans_discard_request(struct request_queue *q,
-                                    struct request *req)
-{
-        req->cmd_type = REQ_TYPE_LINUX_BLOCK;
-        req->cmd[0] = REQ_LB_OP_DISCARD;
-        return 0;
-}
-
 static int do_blktrans_request(struct mtd_blktrans_ops *tr,
                                struct mtd_blktrans_dev *dev,
                                struct request *req)
@@ -52,10 +44,6 @@ static int do_blktrans_request(struct mtd_blktrans_ops *tr,
 
         buf = req->buffer;
 
-        if (req->cmd_type == REQ_TYPE_LINUX_BLOCK &&
-            req->cmd[0] == REQ_LB_OP_DISCARD)
-                return tr->discard(dev, block, nsect);
-
         if (!blk_fs_request(req))
                 return -EIO;
 
@@ -63,6 +51,9 @@ static int do_blktrans_request(struct mtd_blktrans_ops *tr,
             get_capacity(req->rq_disk))
                 return -EIO;
 
+        if (blk_discard_rq(req))
+                return tr->discard(dev, block, nsect);
+
         switch(rq_data_dir(req)) {
         case READ:
                 for (; nsect > 0; nsect--, block++, buf += tr->blksize)
@@ -380,8 +371,8 @@ int register_mtd_blktrans(struct mtd_blktrans_ops *tr)
         tr->blkcore_priv->rq->queuedata = tr;
         blk_queue_logical_block_size(tr->blkcore_priv->rq, tr->blksize);
         if (tr->discard)
-                blk_queue_set_discard(tr->blkcore_priv->rq,
-                                      blktrans_discard_request);
+                queue_flag_set_unlocked(QUEUE_FLAG_DISCARD,
+                                        tr->blkcore_priv->rq);
 
         tr->blkshift = ffs(tr->blksize) - 1;
 
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
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index e23a86cae5ac..f62d45e87618 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);