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

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

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.