Re: [PATCH v15 13/13] block: apply COR-filter to block-stream jobs

[Max Reitz <mreitz@redhat.com>]

On 22.12.20 19:07, Vladimir Sementsov-Ogievskiy wrote:
> 22.12.2020 19:20, Max Reitz wrote:
>> On 16.12.20 07:17, Vladimir Sementsov-Ogievskiy wrote:
>>> From: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
>>>
>>> This patch completes the series with the COR-filter applied to
>>> block-stream operations.
>>>
>>> Adding the filter makes it possible in future implement discarding
>>> copied regions in backing files during the block-stream job, to reduce
>>> the disk overuse (we need control on permissions).
>>>
>>> Also, the filter now is smart enough to do copy-on-read with specified
>>> base, so we have benefit on guest reads even when doing block-stream of
>>> the part of the backing chain.
>>>
>>> Several iotests are slightly modified due to filter insertion.
>>>
>>> Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>>> ---
>>>   block/stream.c             | 105 ++++++++++++++++++++++---------------
>>>   tests/qemu-iotests/030     |   8 +--
>>>   tests/qemu-iotests/141.out |   2 +-
>>>   tests/qemu-iotests/245     |  20 ++++---
>>>   4 files changed, 80 insertions(+), 55 deletions(-)
>>>
>>> diff --git a/block/stream.c b/block/stream.c
>>> index 626dfa2b22..1fa742b0db 100644
>>> --- a/block/stream.c
>>> +++ b/block/stream.c
>>
>> [...]
>>
>>> @@ -266,30 +251,62 @@ void stream_start(const char *job_id, 
>>> BlockDriverState *bs,
>>
>> [...]
>>
>>>       /* Make sure that the image is opened in read-write mode */
>>>       bs_read_only = bdrv_is_read_only(bs);
>>>       if (bs_read_only) {
>>> -        if (bdrv_reopen_set_read_only(bs, false, errp) != 0) {
>>> -            bs_read_only = false;
>>> -            goto fail;
>>> +        int ret;
>>> +        /* Hold the chain during reopen */
>>> +        if (bdrv_freeze_backing_chain(bs, above_base, errp) < 0) {
>>> +            return;
>>> +        }
>>> +
>>> +        ret = bdrv_reopen_set_read_only(bs, false, errp);
>>> +
>>> +        /* failure, or cor-filter will hold the chain */
>>> +        bdrv_unfreeze_backing_chain(bs, above_base);
>>> +
>>> +        if (ret < 0) {
>>
>> Shouldn’t we keep the “bs_read_only = false;” here?
>>
> 
> No, as we don't goto fail.

Ah, right, then it won’t do anything.

> (pre-patch, we goto fail here, and don't want 
> fail: code path to reopend back to RW (as reopening to RO is failed 
> anyway (and we hope it's transactional enough)))

That’s why we had bs_read_only = false; pre-patch, so the reopen back to 
RW is skipped.

And with this patch, we don’t need anything else from the “fail” path 
(freezing is done by the filter, and the filter doesn’t exist yet), so 
it’s correct to condense the “bs_read_only = false; goto fail;” into a 
plain “return”.

Reviewed-by: Max Reitz <mreitz@redhat.com>