nbd: handle discard requests

Add discard support to nbd.  If the nbd-server supports discard, it will
send NBD_FLAG_SEND_TRIM to the client.  The client will then set the flag
in the kernel via NBD_SET_FLAGS, which tells the kernel to enable discards
for the device (QUEUE_FLAG_DISCARD).

If discard support is enabled, then when the nbd client system receives a
discard request, this will be passed along to the nbd-server.  When the
discard request is received by the nbd-server, it will perform:

	fallocate(.. FALLOC_FL_PUNCH_HOLE ..)

To punch a hole in the backend storage, which is no longer needed.

Signed-off-by: Paul Clements <paul.clements@steeleye.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 19 insertions, 2 deletions

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 68fe7514413d..043ddcca4abf 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -98,6 +98,7 @@ static const char *nbdcmd_to_ascii(int cmd)
         case  NBD_CMD_READ: return "read";
         case NBD_CMD_WRITE: return "write";
         case  NBD_CMD_DISC: return "disconnect";
+        case  NBD_CMD_TRIM: return "trim/discard";
         }
         return "invalid";
 }
@@ -469,7 +470,11 @@ static void nbd_handle_req(struct nbd_device *nbd, struct request *req)
 
         nbd_cmd(req) = NBD_CMD_READ;
         if (rq_data_dir(req) == WRITE) {
-                nbd_cmd(req) = NBD_CMD_WRITE;
+                if ((req->cmd_flags & REQ_DISCARD)) {
+                        WARN_ON(!(nbd->flags & NBD_FLAG_SEND_TRIM));
+                        nbd_cmd(req) = NBD_CMD_TRIM;
+                } else
+                        nbd_cmd(req) = NBD_CMD_WRITE;
                 if (nbd->flags & NBD_FLAG_READ_ONLY) {
                         dev_err(disk_to_dev(nbd->disk),
                                 "Write on read-only\n");
@@ -676,6 +681,10 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
 
                 mutex_unlock(&nbd->tx_lock);
 
+                if (nbd->flags & NBD_FLAG_SEND_TRIM)
+                        queue_flag_set_unlocked(QUEUE_FLAG_DISCARD,
+                                nbd->disk->queue);
+
                 thread = kthread_create(nbd_thread, nbd, nbd->disk->disk_name);
                 if (IS_ERR(thread)) {
                         mutex_lock(&nbd->tx_lock);
@@ -693,6 +702,7 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
                 nbd->file = NULL;
                 nbd_clear_que(nbd);
                 dev_warn(disk_to_dev(nbd->disk), "queue cleared\n");
+                queue_flag_clear_unlocked(QUEUE_FLAG_DISCARD, nbd->disk->queue);
                 if (file)
                         fput(file);
                 nbd->bytesize = 0;
@@ -811,6 +821,9 @@ static int __init nbd_init(void)
                  * Tell the block layer that we are not a rotational device
                  */
                 queue_flag_set_unlocked(QUEUE_FLAG_NONROT, disk->queue);
+                disk->queue->limits.discard_granularity = 512;
+                disk->queue->limits.max_discard_sectors = UINT_MAX;
+                disk->queue->limits.discard_zeroes_data = 0;
         }
 
         if (register_blkdev(NBD_MAJOR, "nbd")) {
diff --git a/include/linux/nbd.h b/include/linux/nbd.h
index 4b8ed5a1cfb0..5c86e2b33e2d 100644
--- a/include/linux/nbd.h
+++ b/include/linux/nbd.h
@@ -32,12 +32,16 @@
 enum {
         NBD_CMD_READ = 0,
         NBD_CMD_WRITE = 1,
-        NBD_CMD_DISC = 2
+        NBD_CMD_DISC = 2,
+        /* there is a gap here to match userspace */
+        NBD_CMD_TRIM = 4
 };
 
 /* values for flags field */
 #define NBD_FLAG_HAS_FLAGS    (1 << 0) /* nbd-server supports flags */
 #define NBD_FLAG_READ_ONLY    (1 << 1) /* device is read-only */
+/* there is a gap here to match userspace */
+#define NBD_FLAG_SEND_TRIM    (1 << 5) /* send trim/discard */
 
 #define nbd_cmd(req) ((req)->cmd[0])