Re: [Qemu-devel] [PATCH v6 16/42] block: Flush all children in generic code

[Max Reitz]

On 09.09.19 12:01, Kevin Wolf wrote:
> Am 09.09.2019 um 10:31 hat Max Reitz geschrieben:
>> On 05.09.19 18:24, Kevin Wolf wrote:
>>> Am 12.08.2019 um 14:58 hat Max Reitz geschrieben:
>>>> On 10.08.19 17:36, Vladimir Sementsov-Ogievskiy wrote:
>>>>> 09.08.2019 19:13, Max Reitz wrote:
>>>>>> If the driver does not support .bdrv_co_flush() so bdrv_co_flush()
>>>>>> itself has to flush the children of the given node, it should not flush
>>>>>> just bs->file->bs, but in fact all children.
>>>>>>
>>>>>> In any case, the BLKDBG_EVENT() should be emitted on the primary child,
>>>>>> because that is where a blkdebug node would be if there is any.
>>>>>>
>>>>>> Suggested-by: Vladimir Sementsov-Ogievskiy <address@hidden>
>>>>>> Signed-off-by: Max Reitz <address@hidden>
>>>>>> ---
>>>>>>   block/io.c | 23 +++++++++++++++++------
>>>>>>   1 file changed, 17 insertions(+), 6 deletions(-)
>>>>>>
>>>>>> diff --git a/block/io.c b/block/io.c
>>>>>> index c5a8e3e6a3..bcc770d336 100644
>>>>>> --- a/block/io.c
>>>>>> +++ b/block/io.c
>>>>>> @@ -2572,6 +2572,8 @@ static void coroutine_fn bdrv_flush_co_entry(void 
>>>>>> *opaque)
>>>>>>   
>>>>>>   int coroutine_fn bdrv_co_flush(BlockDriverState *bs)
>>>>>>   {
>>>>>> +    BdrvChild *primary_child = bdrv_primary_child(bs);
>>>>>> +    BdrvChild *child;
>>>>>>       int current_gen;
>>>>>>       int ret = 0;
>>>>>>   
>>>>>> @@ -2601,7 +2603,7 @@ int coroutine_fn bdrv_co_flush(BlockDriverState 
>>>>>> *bs)
>>>>>>       }
>>>>>>   
>>>>>>       /* Write back cached data to the OS even with cache=unsafe */
>>>>>> -    BLKDBG_EVENT(bs->file, BLKDBG_FLUSH_TO_OS);
>>>>>> +    BLKDBG_EVENT(primary_child, BLKDBG_FLUSH_TO_OS);
>>>>>>       if (bs->drv->bdrv_co_flush_to_os) {
>>>>>>           ret = bs->drv->bdrv_co_flush_to_os(bs);
>>>>>>           if (ret < 0) {
>>>>>> @@ -2611,15 +2613,15 @@ int coroutine_fn bdrv_co_flush(BlockDriverState 
>>>>>> *bs)
>>>>>>   
>>>>>>       /* But don't actually force it to the disk with cache=unsafe */
>>>>>>       if (bs->open_flags & BDRV_O_NO_FLUSH) {
>>>>>> -        goto flush_parent;
>>>>>> +        goto flush_children;
>>>>>>       }
>>>>>>   
>>>>>>       /* Check if we really need to flush anything */
>>>>>>       if (bs->flushed_gen == current_gen) {
>>>>>> -        goto flush_parent;
>>>>>> +        goto flush_children;
>>>>>>       }
>>>>>>   
>>>>>> -    BLKDBG_EVENT(bs->file, BLKDBG_FLUSH_TO_DISK);
>>>>>> +    BLKDBG_EVENT(primary_child, BLKDBG_FLUSH_TO_DISK);
>>>>>>       if (!bs->drv) {
>>>>>>           /* bs->drv->bdrv_co_flush() might have ejected the BDS
>>>>>>            * (even in case of apparent success) */
>>>>>> @@ -2663,8 +2665,17 @@ int coroutine_fn bdrv_co_flush(BlockDriverState 
>>>>>> *bs)
>>>>>>       /* Now flush the underlying protocol.  It will also have 
>>>>>> BDRV_O_NO_FLUSH
>>>>>>        * in the case of cache=unsafe, so there are no useless flushes.
>>>>>>        */
>>>>>> -flush_parent:
>>>>>> -    ret = bs->file ? bdrv_co_flush(bs->file->bs) : 0;
>>>>>> +flush_children:
>>>>>> +    ret = 0; > +    QLIST_FOREACH(child, &bs->children, next) {
>>>>>> +        int this_child_ret;
>>>>>> +
>>>>>> +        this_child_ret = bdrv_co_flush(child->bs);
>>>>>> +        if (!ret) {
>>>>>> +            ret = this_child_ret;
>>>>>> +        }
>>>>>> +    }
>>>>>
>>>>> Hmm, you said that we want to flush only children with write-access from 
>>>>> parent..
>>>>
>>>> Good that you remember it, I must have overlooked it (when reading the
>>>> replies to the previous version). :-)
>>>>
>>>>> Shouldn't we check it? Or we assume that it's always safe to call 
>>>>> bdrv_co_flush on
>>>>> a node?
>>>>
>>>> I think it’s always safe.  But checking it seems like a nice touch, yes.
>>>
>>> I'm not sure why we would unconditionally flush all children anyway. The
>>> only drivers I can think of that really need to flush more than one
>>> child are blkverify and quorum, and both of them already implement this.
>>> blkverify implements .bdrv_co_flush, so it's not affected by the change
>>> anyway, but quorum children will be flushed twice now.
>>>
>>> But more than this, I'm worried about the overhead of needlessly
>>> recursing through the whole backing chain and calling flush on every
>>> node there.  Maybe bs->write_gen saves us so that at least this doesn't
>>> result in an fdatasync() call for each, but still... Without a use case,
>>> I'd rather not do this.
>>>
>>> Oh, well, after having written all of this, I see that qcow2 with an
>>> external data file is buggy... This could be fixed in the qcow2 driver,
>>> but maybe restricting the recursion to read-only is actually good enough
>>> then. Can you mention this case in the commit message and maybe build a
>>> test for it?
>>
>> And I should thus probably drop vmdk’s .bdrv_co_flush_to_disk()
>> implementation.
>>
>> I will indeed try to write a test, but to be completely honest, I feel
>> like this series is long enough.
> 
> I guess I could already merge patch 1 to give you space for another test
> patch. ;-)

Don’t forget the mirror-top patch.  And AFAIR, there was some comment
from Vladimir that also required an additional patch.  So it would need
to be three!

(Or I just start squashing from the back?)

Max

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