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

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Tue, May 31, 2016 at 6:42 PM, Wei Liu <wei.liu2@xxxxxxxxxx> wrote:
> From: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>
> 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.
---

 -George

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