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

[Kevin Wolf <kwolf@redhat.com>]

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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 <vsementsov@virtuozzo.com>
> >>>> Signed-off-by: Max Reitz <mreitz@redhat.com>
> >>>> ---
> >>>>   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. ;-)

Kevin

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