Commit 3a2d63f8 authored Feb 27, 2013 by Paolo Bonzini Committed by Linus Torvalds Feb 27, 2013

nbd: fsync and kill block device on shutdown



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@redhat.com>
Acked-by: Paul Clements <Paul.Clements@steeleye.com>
Cc: Alex Bligh <alex@alex.org.uk>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 75f187ab

drivers/block/nbd.c

+10 −0

Original line number	Diff line number	Diff line
		@@ -608,12 +608,20 @@ 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;

		nbd_send_req(nbd, &sreq);
		return 0;
		}
		@@ -627,6 +635,7 @@ 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;
		@@ -719,6 +728,7 @@ 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);
		queue_flag_clear_unlocked(QUEUE_FLAG_DISCARD, nbd->disk->queue);
		if (file)
		fput(file);

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