[PATCH 1/1] nbd: fsync and kill block device on shutdown

[Qiang Huang <h.huangqiang@xxxxxxxxxx>]

From: Paolo Bonzini <pbonzini@xxxxxxxxxx>

commit 3a2d63f87989e01437ba994df5f297528c353d7d upstream.

There are two problems with shutdown in the NBD driver.

1: Receiving the NBD_DISCONNECT ioctl does not sync the filesystem.

   This patch adds the sync operation into __nbd_ioctl()'s
   NBD_DISCONNECT handler.  This is useful because BLKFLSBUF is restricted
   to processes that have CAP_SYS_ADMIN, and the NBD client may not
   possess it (fsync of the block device does not sync the filesystem,
   either).

2: Once we clear the socket we have no guarantee that later reads will
   come from the same backing storage.

   The patch adds calls to kill_bdev() in __nbd_ioctl()'s socket
   clearing code so the page cache is cleaned, lest reads that hit on the
   page cache will return stale data from the previously-accessible disk.

Example:

    # qemu-nbd -r -c/dev/nbd0 /dev/sr0
    # file -s /dev/nbd0
    /dev/stdin: # UDF filesystem data (version 1.5) etc.
    # qemu-nbd -d /dev/nbd0
    # qemu-nbd -r -c/dev/nbd0 /dev/sda
    # file -s /dev/nbd0
    /dev/stdin: # UDF filesystem data (version 1.5) etc.

While /dev/sda has:

    # file -s /dev/sda
    /dev/sda: x86 boot sector; etc.

Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Acked-by: Paul Clements <Paul.Clements@xxxxxxxxxxxx>
Cc: Alex Bligh <alex@xxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.2:
 - Adjusted context
 - s/\bnbd\b/lo/
 - Incorporate export of kill_bdev() from commit ff01bb483265
   ('fs: move code out of buffer.c')]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
[hq: Backported to 3.4: Adjusted context]
Signed-off-by: Qiang Huang <h.huangqiang@xxxxxxxxxx>
---
 drivers/block/nbd.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index d724da5..35fc569 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -584,10 +584,17 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
 		struct request sreq;

 		dev_info(disk_to_dev(nbd->disk), "NBD_DISCONNECT\n");
+		if (!nbd->sock)
+			return -EINVAL;

+		mutex_unlock(&nbd->tx_lock);
+		fsync_bdev(bdev);
+		mutex_lock(&nbd->tx_lock);
 		blk_rq_init(NULL, &sreq);
 		sreq.cmd_type = REQ_TYPE_SPECIAL;
 		nbd_cmd(&sreq) = NBD_CMD_DISC;
+
+		/* Check again after getting mutex back.  */
 		if (!nbd->sock)
 			return -EINVAL;

@@ -606,6 +613,7 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
 		nbd_clear_que(nbd);
 		BUG_ON(!list_empty(&nbd->queue_head));
 		BUG_ON(!list_empty(&nbd->waiting_queue));
+		kill_bdev(bdev);
 		if (file)
 			fput(file);
 		return 0;
@@ -688,6 +696,7 @@ 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");
+		kill_bdev(bdev);
 		if (file)
 			fput(file);
 		nbd->bytesize = 0;
-- 1.8.3

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