Re: [PATCH v4 2/6] block: Allow changing bs->file on reopen

[Kevin Wolf]

Am 07.05.2021 um 09:11 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 17.03.2021 20:15, Alberto Garcia wrote:
> > When the x-blockdev-reopen was added it allowed reconfiguring the
> > graph by replacing backing files, but changing the 'file' option was
> > forbidden. Because of this restriction some operations are not
> > possible, notably inserting and removing block filters.
> 
> 
> I now started to work on making backup-top filter public..
> 
> And I think, we'll need separate commands to insert/remove filters
> anyway.. As blockdev-reopen has the following problems:
> 
> 1. It can't append filter above top node, connected to block-device.
> (but bdrv_append() can do it)

We once had some patches that made the 'drive' qdev property runtime
writable. What happened to them?

> 2. It can't simultaneously create new node and append it. This is
> important for backup-top filter, which unshares write even when has no
> writing parent. Now bdrv_append() works in a smart way for it: it
> first do both graph modification (add child to filter, and replace
> original node by filter) and then update graph permissions. So, we'll
> need a command which in one roll create filter node and inserts it
> where needed.

What backup-top could do, however, is enabling restrictions only if it
has a parent (no matter whether that parent requires writing or not).

> 3. blockdev-reopen requires to specify all options (otherwise, they
> will be changed to default). So it requires passing a lot of
> information. But we don't need to touch any option of original bs
> parent to insert a filter between parent and bs. In other words, we
> don't want to reopen something. We want to insert filter.

Yeah, but this was a decision we made consciously because otherwise we'd
have a hard time telling which options should be updated and which
shouldn't, and we would need separate QAPI types for open and reopen.

If we now say that this is a reason for avoiding blockdev-reopen even
though changing some option is the goal, that would be inconsistent.

> 
> ===
> 
> Hmm, another mentioned use of blockdev-reopen was reopening some RO
> node to RW, to modify bitmaps.. And here again, blockdev-reopen is not
> very convenient:
> 
> 1. Again, it requires to specify all options (at least, that was not
> default on node open).. And this only to change one property:
> read-only. Seems overcomplicated.
> 
> 2. Bitmaps modifications are usually done in transactions. Adding a
> clean transaction support for small command that reopens only to RW,
> and back to RO on transaction finalization seems simpler, than for
> entire generic blockdev-reopen.
> 
> 
> ===
> 
> Hmm, interesting. x-blockdev-reopen says that not specified options
> are reset to default. x-blockdev-reopen works through
> bdrv_reopen_multiple, so I think bdrv_reopen_mutliple should reset
> options to default as well. Now look at bdrv_reopen_set_read_only():
> it specifies only one option: "read-only". This means that all other
> options will be reset to default. But for sure, callers of
> bdrv_reopen_set_read_only() want only to change read-only status of
> node, not all other options. Do we have a bug here?

The difference between these cases is the keep_old_opts parameter to
bdrv_reopen_queue(). It is false for x-blockdev-reopen, but true in
bdrv_reopen_set_read_only().

Kevin