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Re: [PATCH 02/12] libxenguest: deal with log-dirty op stats overflow

  • To: Olaf Hering <olaf@xxxxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Mon, 28 Jun 2021 13:20:06 +0200
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  • Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Ian Jackson <iwj@xxxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Mon, 28 Jun 2021 11:20:17 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 28.06.2021 13:10, Olaf Hering wrote:
> Am Mon, 28 Jun 2021 09:48:26 +0200
> schrieb Jan Beulich <jbeulich@xxxxxxxx>:
>> On 25.06.2021 18:36, Andrew Cooper wrote:
>>> This is an external interface, and I'm not sure it will tolerate finding
>>> more than p2m_size allegedly dirty.  
>> But you do realize that a few lines down from here there already was
>>         policy_stats->dirty_count   = -1;
>> ? Or are you trying to tell me that -1 (documented as indicating
>> "unknown") is okay on subsequent iterations, but not on the first one?
> precopy_policy() gets called twice during each iteration.
> Last time I tried to use this API it was difficult to work with.
> It is required to look at dirty_count and iteration to see the actual state.
> Maybe it was just me who initially failed to fully understand the intent.
> I think as it is right now, the first run with iteration being zero is
> the only way to know the actual p2m_size, in case the consumer really
> wants to know this detail.

But if a field named dirty_count was intended to convey the P2M size
(and then only on the first iteration), this very certainly would
have needed writing down somewhere.




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