Re: [PATCH 2/3] nbd: fsync and kill block device on shutdown

Andrew Morton
2013-02-12T21:41:58Z

On Tue, 12 Feb 2013 17:06:10 +0100
Paolo Bonzini <pbonzini@redhat.com> wrote:

> There are two problems with shutdown in the NBD driver. The first is
> that receiving the NBD_DISCONNECT ioctl does not sync the filesystem;
> 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).

hm, this says that the lack of a sync is "useful". I think you mean
that the patch-which-adds-the-sync is the thing which is useful, yes?

> The second is that once we clear the socket we have no guarantee that
> later reads will come from the same backing storage. Thus the page cache
> must be cleaned, lest reads that hit on the page cache will return stale
> data from the previously-accessible disk.

That sounds like a problem.

> 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.

OK, we've described the problems but there's no description here of how
the patch addresses those problems.

How does this look?

: 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.