Re: ZONE_DMA

["Martin J. Bligh" <mbligh@mbligh.org>]

Christoph Lameter wrote:
> On Wed, 20 Sep 2006, Martin J. Bligh wrote:
>
> > Having something that's used in generic code that means random
> > things on different arches just seems like a recipe for disaster
> > to me.
>
> ZONE_DMA is only used as ZONE_NORMAL if the architecture does not 
> need ZONE_NORMAL because all of memory is reachable via DMA.

That's still inconsistent because it doesn't say DMA for *which*
device.

> > OK ... but requesting ZONE_DMA means what? DMAable for which device?
> > Is it always a floppy disk? on some platforms a PCI card? And how
> > is the VM meant to know what the device is capable of anyway?
>
> I already explained that twice to you.

We seem to be miscommunicating ... you did indeed give a technically
correct definition. But in practice, AFAICS, it's useless. The requestor
has no idea what the arch has implemented, if it's a driver from
arch-independent code.

> I think we all agree that the situation could be better.

Indeed, that would seem to cause little dispute.

> > Having an arch-specific definition of the limit is arbitrary and
> > useless, is it not? The limit is imposed by the device and its
> > driver, we're not communicating it into any sensible way into the
> > VM code, AFAICS. Unless we're pretending we never call it from
> > generic code, which seems woefully unlikely to me.
>
> Its bad but its not useless. See how various arches use it.
>
> > Are we redefining ZONE_DMA to always be 16MB limit across all
> > architectures? At least that'd be consistent.
>
> That wont work because many architectures use different limits. Maybe you 
> should once in a while have a look at the sources.

I'm perfectly well aware that it's inconsistent, that's my whole point.
However, by some chance of history, it's sort of vaguely working. I
think it's dangerous to mess with it rather than fixing it properly.

AFAICS, the correct way to do this is have the requestor pass a memory
bound into the allocator, and have the arch figure out which zones
are applicable.

> Actually the desaster is cleaned up by this patch. A couple of 
> architectures that were wrongly using ZONE_DMA now use ZONE_NORMAL.

Odd that the PPC64 maintainers didn't seem to know about this.
Perhaps it might be a good idea to talk to them before doing this?

M.

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