Re: [PATCH for-4.7] libxl: keep PoD target adjustment by memory fudge after reload_domain_config()

[Vitaly Kuznetsov <vkuznets@redhat.com>]

George Dunlap <George.Dunlap@eu.citrix.com> writes:

> On Tue, May 31, 2016 at 6:42 PM, Wei Liu <wei.liu2@citrix.com> wrote:
>> From: Vitaly Kuznetsov <vkuznets@redhat.com>
>>
>> Commit 56fb5fd623 ("libxl: adjust PoD target by memory fudge, too")
>> introduced target_memkb adjustment for HVM PoD domains on create. The
>> adjustment is however being reset on reload_domain_config() (e.g. when
>> we reboot the guest).
>
> It was determined that all of the following...
>
>>  For example:
>>
>> I'm trying to create HVM PoD domain with the following settings:
>> memory=1024
>> maxmem=4096
>> When the domain boots for the first time I see the following in Xen:
>>
>> (XEN)     nr_pages=262145 ... max_pages=262400 (looks good)
>>
>> When I reboot the domain I see the following:
>> (XEN)     nr_pages=261889 ... max_pages=262144  (oops)
>>
>> After second reboot:
>> (XEN)     nr_pages=261633 ... max_pages=261888 (even less)
>>
>> And eventually the domain crashes with "p2m_pod_demand_populate: Dom4 out of
>> PoD memory! (tot=261377 ents=787200 dom4)".
>
> has actually nothing to do with this patch; and that actually,
> contrary to the description above, the problem is that the adjustment
> is *not* being reset.
>
> A better description would be something like this:
>
> ---
> Commit 56fb5fd623 ("libxl: adjust PoD target by memory fudge, too")
> introduced target_memkb adjustment for HVM PoD domains on create,
> wherein
> the value it wrote to target is always 1MiB lower than the actual
> target_memkb.  Unfortunately, on reboot, it is this value which is
> read *unmodified* to feed into the next domain creation; from which
> 1MiB is subtracted *again*.  This means that any guest which reboots
> with memory < maxmem will have its memory target decreased by 1MiB on
> every boot.
>
> This patch makes it so that when reading target on reboot, we adjust
> the value we read *up* by 1MiB, so that the domain will be build with
> the appropriate amount of memory and the target will remain the same
> after reboot.
>
> This is still not quite a complete fix, as the 1MiB offset is only
> subtracted when creating or rebooting; it is not subtracted when 'xl
> set-memory' is called.  But it will prevent any situations where
> memory is continually increased or decreased.  A better fix will have
> to wait until after the release.
> ---

Yes, thanks, this description is better. Wei, please let me know if you
want me to re-send the patch with this description or if it's possible
to make the adjustment upon commit.

-- 
  Vitaly

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