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

[Andrew Cooper <andrew.cooper3@citrix.com>]

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

>
> Wei.
> ----8<---
> diff --git a/tools/libxl/libxl_dom.c b/tools/libxl/libxl_dom.c
> index 356f920..fb7965d 100644
> --- a/tools/libxl/libxl_dom.c
> +++ b/tools/libxl/libxl_dom.c
> @@ -235,7 +235,7 @@ int libxl__build_pre(libxl__gc *gc, uint32_t domid,
>      libxl_domain_set_nodeaffinity(ctx, domid, &info->nodemap);
>      libxl_set_vcpuaffinity_all(ctx, domid, info->max_vcpus, &info->cpumap);
>  
> -    xc_domain_setmaxmem(ctx->xch, domid, info->target_memkb + LIBXL_MAXMEM_CONSTANT);
> +    xc_domain_setmaxmem(ctx->xch, domid, info->max_memkb + LIBXL_MAXMEM_CONSTANT);
>      xs_domid = xs_read(ctx->xsh, XBT_NULL, "/tool/xenstored/domid", NULL);
>      state->store_domid = xs_domid ? atoi(xs_domid) : 0;
>      free(xs_domid);
>
>
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