Re: [BUG, RFC] Base node is in RW after making external snapshot

[Andrey Drobyshev]

On 4/24/24 21:00, Andrey Drobyshev wrote:
> Hi everyone,
> 
> When making an external snapshot, we end up in a situation when 2 block
> graph nodes related to the same image file (format and storage nodes)
> have different RO flags set on them.
> 
> E.g.
> 
> # ls -la /proc/PID/fd
> lrwx------ 1 root qemu 64 Apr 24 20:14 12 -> /path/to/harddisk.hdd
> 
> # virsh qemu-monitor-command VM '{"execute": "query-named-block-nodes"}'
> --pretty | egrep '"node-name"|"ro"'
>       "ro": false,
>       "node-name": "libvirt-1-format",
>       "ro": false,
>       "node-name": "libvirt-1-storage",
> 
> # virsh snapshot-create-as VM --name snap --disk-only
> Domain snapshot snap created
> 
> # ls -la /proc/PID/fd
> lr-x------ 1 root qemu 64 Apr 24 20:14 134 -> /path/to/harddisk.hdd
> lrwx------ 1 root qemu 64 Apr 24 20:14 135 -> /path/to/harddisk.snap
> 
> # virsh qemu-monitor-command VM '{"execute": "query-named-block-nodes"}'
> --pretty | egrep '"node-name"|"ro"'
>       "ro": false,
>       "node-name": "libvirt-2-format",
>       "ro": false,
>       "node-name": "libvirt-2-storage",
>       "ro": true,
>       "node-name": "libvirt-1-format",
>       "ro": false,                        <--------------
>       "node-name": "libvirt-1-storage",
> 
> File descriptor has been reopened in RO, but "libvirt-1-storage" node
> still has RW permissions set.
> 
> I'm wondering it this a bug or this is intended?  Looks like a bug to
> me, although I see that some iotests (e.g. 273) expect 2 nodes related
> to the same image file to have different RO flags.
> 
> bdrv_reopen_set_read_only()
>   bdrv_reopen()
>     bdrv_reopen_queue()
>       bdrv_reopen_queue_child()
>     bdrv_reopen_multiple()
>       bdrv_list_refresh_perms()
>         bdrv_topological_dfs()
>         bdrv_do_refresh_perms()
>       bdrv_reopen_commit()
> 
> In the stack above bdrv_reopen_set_read_only() is only being called for
> the parent (libvirt-1-format) node.  There're 2 lists: BDSs from
> refresh_list are used by bdrv_drv_set_perm and this leads to actual
> reopen with RO of the file descriptor.  And then there's reopen queue
> bs_queue -- BDSs from this queue get their parameters updated.  While
> refresh_list ends up having the whole subtree (including children, this
> is done in bdrv_topological_dfs()) bs_queue only has the parent.  And
> that is because storage (child) node's (bs->inherits_from == NULL), so
> bdrv_reopen_queue_child() never adds it to the queue.  Could it be the
> source of this bug?
> 
> Anyway, would greatly appreciate a clarification.
> 
> Andrey

Friendly ping.  Could somebody confirm that it is a bug indeed?