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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Wed, Jun 01, 2016 at 10:29:36AM +0200, Vitaly Kuznetsov wrote:
> George Dunlap <George.Dunlap@xxxxxxxxxxxxx> writes:
> 
> > 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.
> > ---
> 
> 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.
> 

No need to resend. I will wait for Ian to review as well and take care
of everything.

Wei.

> -- 
>   Vitaly

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