nbd: support FLUSH requests

Currently, the NBD device does not accept flush requests from the Linux
block layer.  If the NBD server opened the target with neither O_SYNC nor
O_DSYNC, however, the device will be effectively backed by a writeback
cache.  Without issuing flushes properly, operation of the NBD device will
not be safe against power losses.

The NBD protocol has support for both a cache flush command and a FUA
command flag; the server will also pass a flag to note its support for
these features.  This patch adds support for the cache flush command and
flag.  In the kernel, we receive the flags via the NBD_SET_FLAGS ioctl,
and map NBD_FLAG_SEND_FLUSH to the argument of blk_queue_flush.  When the
flag is active the block layer will send REQ_FLUSH requests, which we
translate to NBD_CMD_FLUSH commands.

FUA support is not included in this patch because all free software
servers implement it with a full fdatasync; thus it has no advantage over
supporting flush only.  Because I [Paolo] cannot really benchmark it in a
realistic scenario, I cannot tell if it is a good idea or not.  It is also
not clear if it is valid for an NBD server to support FUA but not flush.
The Linux block layer gives a warning for this combination, the NBD
protocol documentation says nothing about it.

The patch also fixes a small problem in the handling of flags: nbd->flags
must be cleared at the end of NBD_DO_IT, but the driver was not doing
that.  The bug manifests itself as follows.  Suppose you two different
client/server pairs to start the NBD device.  Suppose also that the first
client supports NBD_SET_FLAGS, and the first server sends
NBD_FLAG_SEND_FLUSH; the second pair instead does neither of these two
things.  Before this patch, the second invocation of NBD_DO_IT will use a
stale value of nbd->flags, and the second server will issue an error every
time it receives an NBD_CMD_FLUSH command.

This bug is pre-existing, but it becomes much more important after this
patch; flush failures make the device pretty much unusable, unlike

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Bligh <alex@alex.org.uk>
Acked-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>

1 files changed, 20 insertions, 2 deletions

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index ade146bf65e5..695c68fedd32 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_FLUSH: return "flush";
         case  NBD_CMD_TRIM: return "trim/discard";
         }
         return "invalid";
@@ -244,8 +245,15 @@ static int nbd_send_req(struct nbd_device *nbd, struct request *req)
 
         request.magic = htonl(NBD_REQUEST_MAGIC);
         request.type = htonl(nbd_cmd(req));
-        request.from = cpu_to_be64((u64)blk_rq_pos(req) << 9);
-        request.len = htonl(size);
+
+        if (nbd_cmd(req) == NBD_CMD_FLUSH) {
+                /* Other values are reserved for FLUSH requests.  */
+                request.from = 0;
+                request.len = 0;
+        } else {
+                request.from = cpu_to_be64((u64)blk_rq_pos(req) << 9);
+                request.len = htonl(size);
+        }
         memcpy(request.handle, &req, sizeof(req));
 
         dprintk(DBG_TX, "%s: request %p: sending control (%s@%llu,%uB)\n",
@@ -482,6 +490,11 @@ static void nbd_handle_req(struct nbd_device *nbd, struct request *req)
                 }
         }
 
+        if (req->cmd_flags & REQ_FLUSH) {
+                BUG_ON(unlikely(blk_rq_sectors(req)));
+                nbd_cmd(req) = NBD_CMD_FLUSH;
+        }
+
         req->errors = 0;
 
         mutex_lock(&nbd->tx_lock);
@@ -684,6 +697,10 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
                 if (nbd->flags & NBD_FLAG_SEND_TRIM)
                         queue_flag_set_unlocked(QUEUE_FLAG_DISCARD,
                                 nbd->disk->queue);
+                if (nbd->flags & NBD_FLAG_SEND_FLUSH)
+                        blk_queue_flush(nbd->disk->queue, REQ_FLUSH);
+                else
+                        blk_queue_flush(nbd->disk->queue, 0);
 
                 thread = kthread_create(nbd_thread, nbd, nbd->disk->disk_name);
                 if (IS_ERR(thread)) {
@@ -705,6 +722,7 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
                 queue_flag_clear_unlocked(QUEUE_FLAG_DISCARD, nbd->disk->queue);
                 if (file)
                         fput(file);
+                nbd->flags = 0;
                 nbd->bytesize = 0;
                 bdev->bd_inode->i_size = 0;
                 set_capacity(nbd->disk, 0);