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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

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?

Wei.

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