Re: Linux 4.1 reports wrong number of pages to toolstack

[Juergen Gross <jgross@suse.com>]

On 09/04/2015 02:40 AM, Wei Liu wrote:
> Hi David
>
> This issue is exposed by the introduction of migration v2. The symptom is that
> a guest with 32 bit 4.1 kernel can't be restored because it's asking for too
> many pages.
>
> Note that all guests have 512MB memory, which means they have 131072 pages.
>
> Both 3.14 tests [2] [3] get the correct number of pages.  Like:
>
>     xc: detail: max_pfn 0x1ffff, p2m_frames 256
>     ...
>     xc: detail: Memory: 2048/131072    1%
>     ...
>
> However in both 4.1 [0] [1] the number of pages are quite wrong.
>
> 4.1 32 bit:
>
>     xc: detail: max_pfn 0xfffff, p2m_frames 1024
>     ...
>     xc: detail: Memory: 11264/1048576    1%
>     ...
>
> It thinks it has 4096MB memory.
>
> 4.1 64 bit:
>
>     xc: detail: max_pfn 0x3ffff, p2m_frames 512
>     ...
>     xc: detail: Memory: 3072/262144    1%
>     ...
>
> It thinks it has 1024MB memory.
>
> The total number of pages is determined in libxc by calling
> xc_domain_nr_gpfns, which yanks shared_info->arch.max_pfn from
> hypervisor. And that value is clearly touched by Linux in some way.

Sure. shared_info->arch.max_pfn holds the number of pfns the p2m list
can handle. This is not the memory size of the domain.

> I now think this is a bug in Linux kernel. The biggest suspect is the
> introduction of linear P2M.  If you think this is a bug in toolstack,
> please let me know.

I absolutely think it is a toolstack bug. Even without the linear p2m
things would go wrong in case a ballooned down guest would be migrated,
as shared_info->arch.max_pfn would hold the upper limit of the guest
in this case and not the current size.


Juergen