Hi all!

I now work on v3 for "block: update graph permissions update", and I'm at "[PATCH v2 28/36] block: add bdrv_set_backing_noperm() transaction action".

So, the problem is we should handle inherits_from carefully, and most probably it should be updated in bdrv_replace_child_noperm().. And then, bdrv_replace_child_noperm will become a transaction action, which should store old inherits_from to the transaction state for possible rollback.. Or something like this, I didn't try yet. I just thought, may be we can just drop inherits_from?

I decided to learn the thing a bit, and found that the only usage of inherits_from is to limit reopen process. When adding bs to reopen_queue we do add its children recursively, but only those which inherits from the bs.

That works so starting from

commit 67251a311371c4d22e803f151f47fe817175b6c3
Author: Kevin Wolf <kwolf@redhat.com>
Date:   Thu Apr 9 18:54:04 2015 +0200

    block: Fix reopen flag inheritance


The commit made two things:

1. reopen recursively all* children, not only .file. That's OK.

2. * : not all, but only that inherits_from bs.

[2] Means that we don't reopen some implicitely created children.. And, I want to ask, why?

For me it seems that if we have reopen process.. And bs involved. And it has a child.. And child role defines how that child should inherit options.. Why not to just inherit them?


I decided to check iotests with dropped inherits_from feature.

For ./check -qcow2 on tmpfs only three failed: 30, 245, 258, not bad!

30 and 258 are easily fixed by assuming that if filter driver don't have .bdrv_reopen_prepare handler, it default to noop.

For 245 behavior changes in some places but it seems correct to me. And we have to fix commit job to keep reference to its filter node, otherwise we crash when remove the backing link from node to commit-top-filter of underlying commit job, which is allowed now.


I started this text as a letter, but finally I've fixed problems in 245 and decided to send and RFC series. Probably I miss some core sense of inherits_from, so that's an RFC.. So, what do you think?


Vladimir Sementsov-Ogievskiy (3):
  block/commit: keep reference on commit_top_bs
  block: allow filters to be reopened without .bdrv_reopen_prepare
  block: drop inherits_from logic

 include/block/block_int.h  |  4 --
 block.c                    | 95 ++++++--------------------------------
 block/commit.c             |  8 ++--
 tests/qemu-iotests/245     | 36 +++++++++------
 tests/qemu-iotests/245.out |  8 +++-
 5 files changed, 47 insertions(+), 104 deletions(-)

-- 
2.29.2

Am 11.03.2021 um 16:15 hat Vladimir Sementsov-Ogievskiy geschrieben:
> Hi all!
> 
> I now work on v3 for "block: update graph permissions update", and I'm
> at "[PATCH v2 28/36] block: add bdrv_set_backing_noperm() transaction
> action".
> 
> So, the problem is we should handle inherits_from carefully, and most
> probably it should be updated in bdrv_replace_child_noperm().. And
> then, bdrv_replace_child_noperm will become a transaction action,
> which should store old inherits_from to the transaction state for
> possible rollback.. Or something like this, I didn't try yet. I just
> thought, may be we can just drop inherits_from?
> 
> I decided to learn the thing a bit, and found that the only usage of
> inherits_from is to limit reopen process. When adding bs to
> reopen_queue we do add its children recursively, but only those which
> inherits from the bs.
> 
> That works so starting from
> 
> commit 67251a311371c4d22e803f151f47fe817175b6c3
> Author: Kevin Wolf <kwolf@redhat.com>
> Date:   Thu Apr 9 18:54:04 2015 +0200
> 
>     block: Fix reopen flag inheritance
> 
> 
> The commit made two things:
> 
> 1. reopen recursively all* children, not only .file. That's OK.
> 
> 2. * : not all, but only that inherits_from bs.
> 
> [2] Means that we don't reopen some implicitely created children..
> And, I want to ask, why?

The reason is the difference between

    -drive if=none,file=test.qcow2

and something like

    -blockdev file,filename=backing.img,node-name=backing
    -blockdev file,filename=test.qcow2,node-name=file
    -blockdev qcow2,file=file,backing=backing

The former means that bs->file and bs->backing inherit options from the
qcow2 layer. If you reopen the qcow2 layer to set cache.direct=on, both
children inherit the same update and both the file itself and the
backing file will use O_DIRECT - this is the same as would happen if you
had set cache.direct=on in the -drive option from the start.

In the -blockdev case, the nodes were defined explicitly without
inheriting from the qcow2 layer. Setting cache.direct=on on the qcow2
layer (which is actually created last) doesn't influence the two file
layers. So a reopen of the qcow2 layer shouldn't change the two file
nodes either: If they didn't inherit the option during bdrv_open(), they
certainly shouldn't inherit it during bdrv_reopen() either.

> For me it seems that if we have reopen process.. And bs involved. And
> it has a child.. And child role defines how that child should inherit
> options.. Why not to just inherit them?

The -blockdev behaviour makes things a lot more predictable for a
management tool for which we know that it can handle things on the node
level.

So what we really want is not inheriting at all. But compatibility with
-drive doesn't let us. (And actually -blockdev with inline declaration
of children behaves the same as -drive, which may have been a mistake.)

Kevin

11.03.2021 20:09, Kevin Wolf wrote:
> Am 11.03.2021 um 16:15 hat Vladimir Sementsov-Ogievskiy geschrieben:
>> Hi all!
>>
>> I now work on v3 for "block: update graph permissions update", and I'm
>> at "[PATCH v2 28/36] block: add bdrv_set_backing_noperm() transaction
>> action".
>>
>> So, the problem is we should handle inherits_from carefully, and most
>> probably it should be updated in bdrv_replace_child_noperm().. And
>> then, bdrv_replace_child_noperm will become a transaction action,
>> which should store old inherits_from to the transaction state for
>> possible rollback.. Or something like this, I didn't try yet. I just
>> thought, may be we can just drop inherits_from?
>>
>> I decided to learn the thing a bit, and found that the only usage of
>> inherits_from is to limit reopen process. When adding bs to
>> reopen_queue we do add its children recursively, but only those which
>> inherits from the bs.
>>
>> That works so starting from
>>
>> commit 67251a311371c4d22e803f151f47fe817175b6c3
>> Author: Kevin Wolf <kwolf@redhat.com>
>> Date:   Thu Apr 9 18:54:04 2015 +0200
>>
>>      block: Fix reopen flag inheritance
>>
>>
>> The commit made two things:
>>
>> 1. reopen recursively all* children, not only .file. That's OK.
>>
>> 2. * : not all, but only that inherits_from bs.
>>
>> [2] Means that we don't reopen some implicitely created children..
>> And, I want to ask, why?
> 
> The reason is the difference between
> 
>      -drive if=none,file=test.qcow2
> 
> and something like
> 
>      -blockdev file,filename=backing.img,node-name=backing
>      -blockdev file,filename=test.qcow2,node-name=file
>      -blockdev qcow2,file=file,backing=backing
> 
> The former means that bs->file and bs->backing inherit options from the
> qcow2 layer. If you reopen the qcow2 layer to set cache.direct=on, both
> children inherit the same update and both the file itself and the
> backing file will use O_DIRECT - this is the same as would happen if you
> had set cache.direct=on in the -drive option from the start.
> 
> In the -blockdev case, the nodes were defined explicitly without
> inheriting from the qcow2 layer. Setting cache.direct=on on the qcow2
> layer (which is actually created last) doesn't influence the two file
> layers. So a reopen of the qcow2 layer shouldn't change the two file
> nodes either: If they didn't inherit the option during bdrv_open(), they
> certainly shouldn't inherit it during bdrv_reopen() either.

Hmm. Understand. Than I was wrong. So modern blockdev behaviour is NOT inherit options. That makes sense.

> 
>> For me it seems that if we have reopen process.. And bs involved. And
>> it has a child.. And child role defines how that child should inherit
>> options.. Why not to just inherit them?
> 
> The -blockdev behaviour makes things a lot more predictable for a
> management tool for which we know that it can handle things on the node
> level.
> 
> So what we really want is not inheriting at all. But compatibility with
> -drive doesn't let us. (And actually -blockdev with inline declaration
> of children behaves the same as -drive, which may have been a mistake.)
> 

So, inherits_from will disappear together with -drive some good future day. OK, thanks for explanation


-- 
Best regards,
Vladimir