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

[Vladimir Sementsov-Ogievskiy]

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>

> 
>>> +     */
>>> +    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