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Re: [Qemu-block] [PATCH v2 2/3] qcow2: handle reopening bitmaps on bdrv_

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] [PATCH v2 2/3] qcow2: handle reopening bitmaps on bdrv_invalidate_cache
Date: Fri, 22 Dec 2017 19:12:49 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.0

22.12.2017 18:43, Kevin Wolf wrote:
Am 22.12.2017 um 15:25 hat Vladimir Sementsov-Ogievskiy geschrieben:
22.12.2017 16:39, Kevin Wolf wrote:
Am 12.12.2017 um 17:04 hat Vladimir Sementsov-Ogievskiy geschrieben:
Consider migration with shared storage. Persistent bitmaps are stored
on bdrv_inactivate. Then, on destination
process_incoming_migration_bh() calls bdrv_invalidate_cache_all() which
leads to qcow2_load_autoloading_dirty_bitmaps() which fails if bitmaps
are already loaded on destination start. In this case we should call
qcow2_reopen_bitmaps_rw instead.

Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
Reviewed-by: John Snow <address@hidden>
qcow2_invalidate_cache() calls qcow2_close() first, so why are there
still any bitmaps loaded? Isn't this a bug? Do we leak bitmaps when a
qcow2 image is closed?

Interesting point.

Now persistent dirty bitmaps are released at the end of
which is not called here. It was a separate patch

commit 615b5dcf2decbc5f0abb512d13d7e5db2385fa23
Author: Vladimir Sementsov-Ogievskiy <address@hidden>
Date:   Wed Jun 28 15:05:30 2017 +0300

     block: release persistent bitmaps on inactivate

May be it is more correct to release them immediately after storing, in
I chose the question form because I'm really not deep enough into the
bitmap code to have a solid opinion, but I have a feeling that releasing
the bitmaps from the block driver that provided them would be cleaner
indeed. I suppose the same is true for .bdrv_close.

But it will not fix the issue, because qcow2_close will
call qcow2_inactivate only if (!(s->flags & BDRV_O_INACTIVE)), but it is
not the case.
Yes, good point.

Is there a reason why bitmaps are already loaded in qcow2_do_open() even
if the image is inactive? Can bitmaps be meaningfully used on inactive

Otherwise, we could just make qcow2_load_autoloading_dirty_bitmaps()
conditional on cleared BDRV_O_INACTIVE.

or we can do like this, it fixes the new test:
   static void qcow2_close(BlockDriverState *bs)
       BDRVQcow2State *s = bs->opaque;
       /* else pre-write overlap checks in cache_destroy may crash */
       s->l1_table = NULL;

       if (!(s->flags & BDRV_O_INACTIVE)) {
+     bdrv_release_persistent_dirty_bitmaps(bs);

What do you think?
Hm, I think I don't like this much.

We just need to decide what the status of inactive images is supposed to
be. If they should have bitmaps, then your patch is probably right. But
if inactive images shouldn't have any, we need to change qcow2_do_open()
and qcow2_inactivate().


Does Qemu start in inactive mode when and only when it is incoming migration? In this case I don't see any reason of early-load the bitmaps. Backup in inactive mode should not be allowed too, yes?

So, it looks like it's ok to just do not autoload bitmaps if we are in inactive mode. The difference would be that user will not see these bitmaps during migration. And he even may create bitmaps with same names, which will lead to fault. But it don't look like real problem.

So, if there will not be other thoughts, I'll make another patch.

Best regards,

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