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From: Kevin Wolf <kwolf@redhat.com>
To: Max Reitz <mreitz@redhat.com>
Cc: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>,
	"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>,
	"qemu-block@nongnu.org" <qemu-block@nongnu.org>
Subject: Re: [Qemu-devel] [PATCH v6 16/42] block: Flush all children in generic code
Date: Mon, 9 Sep 2019 12:01:47 +0200
Message-ID: <20190909100147.GC17606@localhost.localdomain> (raw)
In-Reply-To: <af64f1db-c35b-351f-d65f-179775010d00@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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  reply index

Thread overview: 136+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-09 16:13 [Qemu-devel] [PATCH v6 00/42] block: Deal with filters Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 01/42] block: Mark commit and mirror as filter drivers Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 02/42] copy-on-read: Support compressed writes Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 03/42] throttle: " Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 04/42] block: Add child access functions Max Reitz
2019-08-09 16:56   ` Eric Blake
2019-09-04 16:16   ` Kevin Wolf
2019-09-09  7:56     ` Max Reitz
2019-09-09  9:36       ` Kevin Wolf
2019-09-09 14:04         ` Max Reitz
2019-09-09 16:13           ` Kevin Wolf
2019-09-10  9:14             ` Max Reitz
2019-09-10 10:47               ` Kevin Wolf
2019-09-10 11:36                 ` Max Reitz
2019-09-10 12:48                   ` Kevin Wolf
2019-09-10 12:59                     ` Max Reitz
2019-09-10 13:10                       ` Kevin Wolf
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 05/42] block: Add chain helper functions Max Reitz
2019-08-09 17:01   ` Eric Blake
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 06/42] qcow2: Implement .bdrv_storage_child() Max Reitz
2019-08-09 17:07   ` Eric Blake
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 07/42] block: *filtered_cow_child() for *has_zero_init() Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 08/42] block: bdrv_set_backing_hd() is about bs->backing Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 09/42] block: Include filters when freezing backing chain Max Reitz
2019-08-10 13:32   ` Vladimir Sementsov-Ogievskiy
2019-08-12 12:56     ` Max Reitz
2019-09-05 13:05   ` Kevin Wolf
2019-09-09  8:02     ` Max Reitz
2019-09-09  9:40       ` Kevin Wolf
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 10/42] block: Drop bdrv_is_encrypted() Max Reitz
2019-08-10 13:42   ` Vladimir Sementsov-Ogievskiy
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 11/42] block: Add bdrv_supports_compressed_writes() Max Reitz
2019-09-05 13:11   ` Kevin Wolf
2019-09-09  8:09     ` Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 12/42] block: Use bdrv_filtered_rw* where obvious Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 13/42] block: Use CAFs in block status functions Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 14/42] block: Use CAFs when working with backing chains Max Reitz
2019-08-10 15:19   ` Vladimir Sementsov-Ogievskiy
2019-09-05 14:05   ` Kevin Wolf
2019-09-09  8:25     ` Max Reitz
2019-09-09  9:55       ` Kevin Wolf
2019-09-09 14:08         ` Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 15/42] block: Re-evaluate backing file handling in reopen Max Reitz
2019-08-10 16:05   ` Vladimir Sementsov-Ogievskiy
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 16/42] block: Flush all children in generic code Max Reitz
2019-08-10 15:36   ` Vladimir Sementsov-Ogievskiy
2019-08-12 12:58     ` Max Reitz
2019-09-05 16:24       ` Kevin Wolf
2019-09-09  8:31         ` Max Reitz
2019-09-09 10:01           ` Kevin Wolf [this message]
2019-09-09 14:15             ` Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 17/42] block: Use CAFs in bdrv_refresh_limits() Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 18/42] block: Use CAFs in bdrv_refresh_filename() Max Reitz
2019-08-10 16:22   ` Vladimir Sementsov-Ogievskiy
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 19/42] block: Use CAF in bdrv_co_rw_vmstate() Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 20/42] block/snapshot: Fix fallback Max Reitz
2019-08-10 16:34   ` Vladimir Sementsov-Ogievskiy
2019-08-12 13:06     ` Max Reitz
2019-09-10 11:56   ` Kevin Wolf
2019-09-10 12:04     ` Max Reitz
2019-09-10 12:49       ` Kevin Wolf
2019-09-10 13:06         ` Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 21/42] block: Use CAFs for debug breakpoints Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 22/42] block: Fix bdrv_get_allocated_file_size's fallback Max Reitz
2019-08-10 16:41   ` Vladimir Sementsov-Ogievskiy
2019-08-12 13:09     ` Max Reitz
2019-08-12 17:14       ` Vladimir Sementsov-Ogievskiy
2019-08-12 19:15         ` Max Reitz
2019-09-10 14:52   ` Kevin Wolf
2019-09-11  6:20     ` Max Reitz
2019-09-11  6:55       ` Kevin Wolf
2019-09-11  7:37         ` Max Reitz
2019-09-11  8:27           ` Kevin Wolf
2019-09-11 10:00             ` Max Reitz
2019-09-11 10:31               ` Kevin Wolf
2019-09-11 11:00                 ` Max Reitz
2019-09-12 10:34                   ` Kevin Wolf
2019-11-14 13:11                   ` Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 23/42] blockdev: Use CAF in external_snapshot_prepare() Max Reitz
2019-09-10 15:02   ` Kevin Wolf
2019-09-11  6:21     ` Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 24/42] block: Use child access functions for QAPI queries Max Reitz
2019-08-10 16:57   ` Vladimir Sementsov-Ogievskiy
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 25/42] mirror: Deal with filters Max Reitz
2019-08-12 11:09   ` Vladimir Sementsov-Ogievskiy
2019-08-12 13:26     ` Max Reitz
2019-08-14 15:17       ` Vladimir Sementsov-Ogievskiy
2019-08-31  9:57   ` Vladimir Sementsov-Ogievskiy
2019-09-02 14:35     ` Max Reitz
2019-09-03  8:32       ` Vladimir Sementsov-Ogievskiy
2019-09-09  7:41         ` Max Reitz
2019-09-13 12:55   ` Kevin Wolf
2019-09-16 10:26     ` Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 26/42] backup: " Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 27/42] commit: " Max Reitz
2019-08-31 10:44   ` Vladimir Sementsov-Ogievskiy
2019-09-02 14:55     ` Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 28/42] stream: " Max Reitz
2019-08-12 11:55   ` Vladimir Sementsov-Ogievskiy
2019-09-13 14:16   ` Kevin Wolf
2019-09-16  9:52     ` Max Reitz
2019-09-16 14:47       ` Kevin Wolf
2019-12-11 12:52       ` Max Reitz
2019-12-11 15:52         ` Kevin Wolf
2019-12-11 16:12           ` Max Reitz
2019-12-11 16:35             ` Kevin Wolf
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 29/42] nbd: Use CAF when looking for dirty bitmap Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 30/42] qemu-img: Use child access functions Max Reitz
2019-08-12 12:14   ` Vladimir Sementsov-Ogievskiy
2019-08-12 13:28     ` Max Reitz
2019-08-14 16:04   ` Vladimir Sementsov-Ogievskiy
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 31/42] block: Drop backing_bs() Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 32/42] block: Make bdrv_get_cumulative_perm() public Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 33/42] blockdev: Fix active commit choice Max Reitz
2019-08-09 16:13 ` [Qemu-devel] [PATCH v6 34/42] block: Inline bdrv_co_block_status_from_*() Max Reitz
2019-08-09 16:14 ` [Qemu-devel] [PATCH v6 35/42] block: Fix check_to_replace_node() Max Reitz
2019-08-15 15:21   ` Vladimir Sementsov-Ogievskiy
2019-08-15 17:01     ` Max Reitz
2019-08-16 11:01       ` Vladimir Sementsov-Ogievskiy
2019-08-16 13:30         ` Max Reitz
2019-08-16 14:24           ` Vladimir Sementsov-Ogievskiy
2019-08-09 16:14 ` [Qemu-devel] [PATCH v6 36/42] iotests: Add tests for mirror @replaces loops Max Reitz
2019-08-09 16:14 ` [Qemu-devel] [PATCH v6 37/42] block: Leave BDS.backing_file constant Max Reitz
2019-08-16 16:16   ` Vladimir Sementsov-Ogievskiy
2019-08-09 16:14 ` [Qemu-devel] [PATCH v6 38/42] iotests: Let complete_and_wait() work with commit Max Reitz
2019-08-23  5:59   ` Vladimir Sementsov-Ogievskiy
2019-08-09 16:14 ` [Qemu-devel] [PATCH v6 39/42] iotests: Add filter commit test cases Max Reitz
2019-08-31 11:41   ` Vladimir Sementsov-Ogievskiy
2019-09-02 15:06     ` Max Reitz
2019-08-31 12:35   ` Vladimir Sementsov-Ogievskiy
2019-09-02 15:09     ` Max Reitz
2019-08-09 16:14 ` [Qemu-devel] [PATCH v6 40/42] iotests: Add filter mirror " Max Reitz
2019-08-31 12:35   ` Vladimir Sementsov-Ogievskiy
2019-08-09 16:14 ` [Qemu-devel] [PATCH v6 41/42] iotests: Add test for commit in sub directory Max Reitz
2019-08-09 16:14 ` [Qemu-devel] [PATCH v6 42/42] iotests: Test committing to overridden backing Max Reitz
2019-09-03  9:18   ` Vladimir Sementsov-Ogievskiy

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