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Re: [Qemu-devel] safety of migration_bitmap_extend
From: |
Juan Quintela |
Subject: |
Re: [Qemu-devel] safety of migration_bitmap_extend |
Date: |
Tue, 03 Nov 2015 13:55:28 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux) |
"Dr. David Alan Gilbert" <address@hidden> wrote:
> Hi,
> I'm trying to understand why migration_bitmap_extend is correct/safe;
> If I understand correctly, you're arguing that:
>
> 1) the migration_bitmap_mutex around the extend, stops any sync's happening
> and so no new bits will be set during the extend.
>
> 2) If migration sends a page and clears a bitmap entry, it doesn't
> matter if we lose the 'clear' because we're copying it as
> we extend it, because losing the clear just means the page
> gets resent, and so the data is OK.
>
> However, doesn't (2) mean that migration_dirty_pages might be wrong?
> If a page was sent, the bit cleared, and migration_dirty_pages decremented,
> then if we copy over that bitmap and 'set' that bit again then
> migration_dirty_pages
> is too small; that means that either migration would finish too early,
> or more likely, migration_dirty_pages would wrap-around -ve and
> never finish.
>
> Is there a reason it's really safe?
No. It is reasonably safe. Various values of reasonably.
migration_dirty_pages should never arrive at values near zero. Because
we move to the completion stage way before it gets a value near zero.
(We could have very, very bad luck, as in it is not safe).
Now, do we really care if migration_dirty_pages is exact? Not really,
we just use it to calculate if we should start the throotle or not.
That only test that each 1 second, so if we have written a couple of
pages that we are not accounting for, things should be reasonably safe.
Once told that, I don't know why we didn't catch that problem during
review (yes, I am guilty here). Not sure how to really fix it,
thought. I think that the problem is more theoretical than real, but
....
Thanks, Juan.
>
> Dave
>
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK
- [Qemu-devel] safety of migration_bitmap_extend, Dr. David Alan Gilbert, 2015/11/03
- Re: [Qemu-devel] safety of migration_bitmap_extend,
Juan Quintela <=
- Re: [Qemu-devel] safety of migration_bitmap_extend, Dr. David Alan Gilbert, 2015/11/03
- Re: [Qemu-devel] safety of migration_bitmap_extend, Wen Congyang, 2015/11/03
- Re: [Qemu-devel] safety of migration_bitmap_extend, Dr. David Alan Gilbert, 2015/11/04
- Re: [Qemu-devel] safety of migration_bitmap_extend, Wen Congyang, 2015/11/04
- Re: [Qemu-devel] safety of migration_bitmap_extend, Dr. David Alan Gilbert, 2015/11/04
- Re: [Qemu-devel] safety of migration_bitmap_extend, Wen Congyang, 2015/11/12
- Re: [Qemu-devel] safety of migration_bitmap_extend, Li Zhijian, 2015/11/13