Re: [Xen-devel] Bug - Xen 4.3 - xl ignores maxmem setting in domU config file

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

On Tue, Oct 29, 2013 at 10:49:35AM -0400, Konrad Rzeszutek Wilk wrote:
> On Tue, Oct 29, 2013 at 12:08:04PM +0000, Wei Liu wrote:
> > On Tue, Oct 29, 2013 at 11:06:01AM +0000, Andrew Cooper wrote:
> > > On 29/10/13 10:59, Wei Liu wrote:
> > > > On Tue, Oct 29, 2013 at 10:26:00AM +0000, James Dingwall wrote:
> > > >> Hi,
> > > >>
> > > >> I am having some memory ballooning problems which only seem to have
> > > >> appeared since upgrading to Xen 4.3.  In short if I set the domU
> > > >> configuration file as:
> > > >>
> > > >> memory = 512
> > > >> maxmem = 1024
> > > >>
> > > >> Then even under memory pressure the guest domain does not balloon past
> > > >> the value of the 'memory' parameter.
> > > >>
> > > >> xl info shows plenty of available memory in dom0:
> > > >> total_memory           : 32767
> > > >> free_memory            : 12445
> > > >>
> > > >>
> > > >> xl list -l on the domain shows:
> > > >>      "max_memkb": 1048576,
> > > >>      "target_memkb": 524288,
> > > >>
> > > > This is parsed from your config file so they should always look OK to
> > > > you.
> > > >
> > > > Does the following patch help?
> > > >
> > > > Git blame tells me the change to set maxmem to target_memkb was
> > > > introduced 4 years ago so I suspect there's reason to do that. If we
> > > > cannot fix it here we need to insert the corresponding call later.
> > >
> > > When setmaxmem sets a limit lower than current, the domain can strictly
> > > only balloon down until it is equal to or under the new limit.
> > >
> > > Performing a setmaxmem hypercall to info->target_memkb will prevent from
> > > domain from ballooning down then back up a bit, when it has been asked
> > > to balloon down a lot; i.e. it must strictly balloon down to the limit
> > > it has been given.
> > >
> > > Therefore, I am not sure this change is valid.
> > >
> > > ~Andrew
> > >
> >
> > Looking at the changeset 9905ac that introduced target_memkb,
> > target_memkb was always set to max_memkb, so my guess is that the
> > purpose was still to use max_memkb to be max memory limit.
> >
> > Probably later changes to that portion of code altered that behavior.
> >
> > The use of target_memkb in xc_domain_setmaxmem looks logically wrong to
> > me anyway...
> >
> > Stefano, thoughts?
>
> Lets also include Daniel in this - as he was trying to fix this in the past.

As I remember this behavior is by design. However, this is not in line with
xm behavior which sometimes makes a problem. I have discussed this with IanJ
and IanC (both CC-ed here) once. I have proposed some patch series to fix that
issue but later I was not able to continue work on it. If you wish I could
repost them once again with fixes requested by IanJ and IanC. You could also
find latest version of patches here:

http://lists.xenproject.org/archives/html/xen-devel/2013-04/msg03072.html

Daniel

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