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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