Re: [Libguestfs] [nbdkit PATCH 3/3] tests: Add regression test for NBDKIT_EMULATE_CACHE fix

On Fri, Jun 10, 2022 at 03:21:17PM -0500, Eric Blake wrote: > On Fri, Jun 10, 2022 at 05:21:19PM +0100, Richard W.M. Jones wrote: > > > > Series looks good: > > > > Acked-by: Richard W.M. Jones <rjones(a)redhat.com&gt; > > > > I think nbdkit-ext2-filter is still wrong (although at least it should > > no longer corrupt disk images) because it unnecessarily calls > > next->can_zero, so it might be worth dropping those two lines. > > ext2 does not have an interface (yet?) for emulating fallocate() > operations (or other bulk-zeroing operation) against a file; if it > _did_ have one, then that's what ext2's .zero should do. But we _do_ > want to allow the compressed transmission effects of > NBD_CMD_WRITE_ZEROES over the network, so having .can_zero return > NBDKIT_ZERO_EMULATE is right. On the other hand, ext2 DOES have a way > for the file system itself to request bulk-zeroing of a portion of the > underlying disk, so we _do_ call into the plugin's next->zero(), which > means we _do_ need to check next->can_zero() up front (see the io.c > callback io_zeroout). I don't see any bugs in this area, once this > series is in. I mean these two lines: https://gitlab.com/nbdkit/nbdkit/-/blob/b4b8bd78ee66e5e1bc3d9b5464b10be07... Before your patch series, they avoid the same assertion fail that we saw in the luks filter. The mechanism is that when ZERO_EMULATE caused filter zero to get forwarded to the plugin, because plugin can_zero had never been called, the assertion fired. Those two lines initialize plugin can_zero, removing the assertion but causing the .zero call to be forwarded to the plugin unmodified (randomly zeroing some part of the disk). After your patch series, the two lines just do nothing at all.

Anyway, I will remove them.