Re: [Qemu-devel] [PATCH v2 00/11] block: Deal with filters

[Eric Blake <eblake@redhat.com>]

On 8/9/18 5:31 PM, Max Reitz wrote:
> Note 1: This series depends on v10 of my “block: Fix some filename
> generation issues” series.
> 
> Based-on: <20180809213528.14738-1-mreitz@redhat.com>
> 
> Note 2: This is technically the first part of my active mirror followup.
> But just very technically.  I noticed that that followup started to
> consist of two parts, namely (A) fix filtery things in the block layer,
> and (B) fix active mirror.  So I decided to split it.  This is part A.
> Part B comes later.
> 
> 
> When we introduced filters, we did it a bit casually.  Sure, we talked a
> lot about them before, but that was mostly discussion about where
> implicit filters should be added to the graph (note that we currently
> only have two implicit filters, those being mirror and commit).  But in
> the end, we really just designated some drivers filters (Quorum,
> blkdebug, etc.) and added some specifically (throttle, COR), without
> really looking through the block layer to see where issues might occur.
> 
> It turns out vast areas of the block layer just don't know about filters
> and cannot really handle them.  Many cases will work in practice, in
> others, well, too bad, you cannot use some feature because some part
> deep inside the block layer looks at your filters and thinks they are
> format nodes.
> 
> This series sets out to correct a bit of that.  I lost my head many
> times and I'm sure this series is incomplete in many ways, but it really
> doesn't do any good if it sits on my disk any longer, it needs to go out
> now.

Is there any of this series that still needs to go in 3.1?

I'm trying to fix an NBD bug with incorrect alignment in response to 
NBD_CMD_BLOCK_STATUS.  I wanted to use blkdebug to force a 
larger-than-512 minimum alignment exposure to the guest.  But right now, 
nbd/server.c:nbd_export_bitmap() does a braindead:

     while (true) {
         bm = bdrv_find_dirty_bitmap(bs, bitmap);
         if (bm != NULL || bs->backing == NULL) {
             break;
         }

         bs = bs->backing->bs;
     }

which fails to see through a blkdebug filter to find a dirty bitmap in 
the underlying qcow2 BDS (because blkdebug uses bs->file, not 
bs->backing).  It looks like your series will make my task a lot easier, 
but if it's not going in for 3.1, I still need to push my bug fix now, 
and defer the testsuite additions to later when we can more sanely see 
through filters.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org