Re: [Xen-devel] 3.0.2-testing: pci_set_dma_mask, pci_set_consistent_dma_mask(pci, 0x0fffffff) returns < 0 (ICE1712)

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 12 Apr 2006, at 19:53, Stephen C. Tweedie wrote:

> On raw metal, when we start to get low on a specific memory zone, 
> either
> for DMA24/DMA32 or on a specific NUMA node, we can start to 
> specifically
> reclaim pages from that memory zone, swapping them out or simply
> evicting cache.
>
> If the Xen HV runs out of MEMZONE_DMADOM pages, aren't we basically out
> of luck right now?  Xen guests can't see that shortage, nor does the
> vmscan.c code have any code to target pages for stealing based on MFN
> rather than PFN.
Yes, if that happens then we could be in trouble. Although mostly low 
memory is allocated for devices only at start of day. The main fly in 
the ointment is PAE pgds. We get around that right now by reserving a 
lowmem pool in Xen that normal allocations cannot fall back to. That 
sorts out most kinds of bad behaviour. We would need the guest kernels 
to help out with reclamation (at least for guests not on shadow page 
tables). If Linux had support for hotplug memory (add and remove) I 
suspect we could make use of that to help out, if we were careful.
 -- Keir


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