Re: [Qemu-block] [PATCH v5 33/42] blockdev: Fix active commit choice

[Vladimir Sementsov-Ogievskiy]

21.06.2019 16:26, Vladimir Sementsov-Ogievskiy wrote:
> 19.06.2019 18:59, Max Reitz wrote:
>> On 19.06.19 11:31, Vladimir Sementsov-Ogievskiy wrote:
>>> 13.06.2019 1:09, Max Reitz wrote:
>>>> We have to perform an active commit whenever the top node has a parent
>>>> that has taken the WRITE permission on it.
>>>>
>>>> Signed-off-by: Max Reitz <address@hidden>
>>>> ---
>>>>    blockdev.c | 24 +++++++++++++++++++++---
>>>>    1 file changed, 21 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/blockdev.c b/blockdev.c
>>>> index a464cabf9e..5370f3b738 100644
>>>> --- a/blockdev.c
>>>> +++ b/blockdev.c
>>>> @@ -3294,6 +3294,7 @@ void qmp_block_commit(bool has_job_id, const char 
>>>> *job_id, const char *device,
>>>>         */
>>>>        BlockdevOnError on_error = BLOCKDEV_ON_ERROR_REPORT;
>>>>        int job_flags = JOB_DEFAULT;
>>>> +    uint64_t top_perm, top_shared;
>>>>        if (!has_speed) {
>>>>            speed = 0;
>>>> @@ -3406,14 +3407,31 @@ void qmp_block_commit(bool has_job_id, const char 
>>>> *job_id, const char *device,
>>>>            goto out;
>>>>        }
>>>> -    if (top_bs == bs) {
>>>> +    /*
>>>> +     * Active commit is required if and only if someone has taken a
>>>> +     * WRITE permission on the top node.  Historically, we have always
>>>> +     * used active commit for top nodes, so continue that practice.
>>>> +     * (Active commit is never really wrong.)
>>>
>>> Hmm, if we start active commit when nobody has write access, than
>>> we leave a possibility to someone to get this access during commit.
>>
>> Isn’t that blocked by the commit filter?  If it isn’t, it should be.
>>
>>> And during
>>> passive commit write access is blocked. So, may be right way is do active 
>>> commit
>>> always? Benefits:
>>> 1. One code path. and it shouldn't be worse when no writers, without guest 
>>> writes
>>> mirror code shouldn't work worse than passive commit, if it is, it should 
>>> be fixed.
>>> 2. Possibility of write access if user needs it during commit
>>> 3. I'm sure that active commit (mirror code) actually works faster, as it 
>>> uses
>>> async requests and smarter handling of block status.
>>
>> Disadvantage: Something may break because the basic commit job does not
>> emit BLOCK_JOB_READY and thus does not require block-job-complete.
>>
>> Technically everything should expect jobs to potentially emit
>> BLOCK_JOB_READY, but who knows.  I think we’d want at least a
>> deprecation period.
>>
>> Max
> 
> OK, so this is for future.. Then:
> 
> Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>

Strange, I have this mail automatically returned back. Did you receive it?

> 
>>
>>>> +     */
>>>> +    bdrv_get_cumulative_perm(top_bs, &top_perm, &top_shared);
>>>> +    if (top_perm & BLK_PERM_WRITE ||
>>>> +        bdrv_skip_rw_filters(top_bs) == bdrv_skip_rw_filters(bs))
>>>> +    {
>>>>            if (has_backing_file) {
>>>>                error_setg(errp, "'backing-file' specified,"
>>>>                                 " but 'top' is the active layer");
>>>>                goto out;
>>>>            }
>>>> -        commit_active_start(has_job_id ? job_id : NULL, bs, base_bs,
>>>> -                            job_flags, speed, on_error,
>>>> +        if (!has_job_id) {
>>>> +            /*
>>>> +             * Emulate here what block_job_create() does, because it
>>>> +             * is possible that @bs != @top_bs (the block job should
>>>> +             * be named after @bs, even if @top_bs is the actual
>>>> +             * source)
>>>> +             */
>>>> +            job_id = bdrv_get_device_name(bs);
>>>> +        }
>>>> +        commit_active_start(job_id, top_bs, base_bs, job_flags, speed, 
>>>> on_error,
>>>>                                filter_node_name, NULL, NULL, false, 
>>>> &local_err);
>>>>        } else {
>>>>            BlockDriverState *overlay_bs = bdrv_find_overlay(bs, top_bs);
>>>>
>>>
>>>
>>
>>
> 
> 


-- 
Best regards,
Vladimir