On Tue, 12 Feb 2013 17:06:10 +0100 Paolo Bonzini <pbonz...@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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
