[PATCH RFC 0/3] block: drop inherits_from

[Vladimir Sementsov-Ogievskiy]

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(-)

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2.29.2