Re: [Xen-devel] Xen x86 host memory limit issues

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.08.15 at 12:36, <andrew.cooper3@xxxxxxxxxx> wrote:
> The infrastructure around xenheap_max_mfn() is supposed cause all
> xenheap page allocations to fall within the Xen direct mapped region,
> but experimentally doesn't work correctly.
> 
> In all cases I have seen, the bad xenheap allocations have been from
> calls which contain numa information in the memflags, which leads me to
> suspect it is an interaction issue of numa hinting information and
> xenheap_bits.  At a guess I suspect alloc_heap_pages() doesn't correctly
> override the numa hint when both a numa hint and zone limit are
> provided, but I have not investigated this yet.

But you're in the ideal position to do so. As said previously on the same
topic, looking just at the code I can't see what's wrong, even when
taking into account the experimentally observed behavior.

> Fixing that bug will be a useful step, as it will allow Xen to function
> with host ram above the direct map limit, but is still not an optimal
> solution as it prevents getting numa-local xenheap memory.
> 
> Longterm it would be optimal to segment the direct map region by numa
> node so there is equal quantities of xenheap memory available from each
> numa node.

Yes, albeit I'm suspecting there to arise (at least theoretical) issues
on systems with many nodes - the per-node ranges directly mapped
may become unreasonably small (and we may risk exhausting node
0's memory due to not NUMA-tagged allocation requests).

Jan


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