On Fri, 26 Jan 2007, Andrew Morton wrote:
> > The main benefit is a significant simplification of the VM, leading to
> > robust and reliable operations and a reduction of the maintenance
> > headaches coming with the additional zones.
> >
> > If we would introduce the ability of allocating from a range of
> > physical addresses then the need for DMA zones would go away allowing
> > flexibility for device driver DMA allocations and at the same time we get
> > rid of special casing in the VM.
>
> None of this is valid. The great majority of machines out there will
> continue to have the same number of zones. Nothing changes.
All 64 bit machine will only have a single zone if we have such a range
alloc mechanism. The 32bit ones with HIGHMEM wont be able to avoid it,
true. But all arches that do not need gymnastics to access their memory
will be able run with a single zone.
> That's all a real cost, so we need to see *good* benefits to outweigh that
> cost. Thus far I don't think we've seen that.
The real savings is the simplicity of VM design, robustness and
efficiency. We loose on all these fronts if we keep or add useless zones.
The main reason for the recent problems with dirty handling seem to be due
to exactly such a multizone balancing issues involving ZONE_NORMAL and
HIGHMEM. Those problems cannot occur on single ZONE arches (this means
right now on a series of embedded arches, UML and IA64).
Multiple ZONES are a recipie for VM fragility and result in complexity
that is difficult to manage.
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