Re: [discuss] Re: 32-bit dma allocations on 64-bit platforms

[Andrea Arcangeli <andrea@suse.de>]

On Fri, Jun 25, 2004 at 01:48:47AM +1000, Nick Piggin wrote:
> Andrea Arcangeli wrote:
> 
> >
> >why does it fail? note that with the lower_zone_reserve_ratio algorithm I
> >added to 2.4 all dma zone will be reserved for __GFP_DMA allocations so
> >you should have troubles only with 2.6, 2.4 should work fine.
> >
> >So with latest 2.4 it has to fail only if you already allocated 16M with
> >pci_alloc_consistent which sounds unlikely.
> >
> >the fact 2.6 lacks the lower_zone_reserve_ratio algorithm is a different
> >issue, but I'm confortable there's no other possible algorithm to solve
> >this memory balancing problem completely so there's no way around a
> >forward port.
> >
> >well 2.6 has a tiny hack like some older 2.4 that attempts to do what
> >lower_zone_reserve_ratio does, but it's not nearly enough, there's no
> >per-zone-point-of-view watermark in 2.6 etc.. 2.6 actually has a more
> >hardcoded hack for highmem, but the lower_zone_reserve_ratio has
> >absolutely nothing to do with highmem vs lowmem. it's by pure
> >coincidence that it avoids highmem machine to lockup without swap, but
> >the very same problem happens on x86-64 with lowmem vs dma.
> 
> 2.6 has the "incremental min" thing. What is wrong with that?
> Though I think it is turned off by default.

sysctl_lower_zone_protection is an inferior implementation of the
lower_zone_reserve_ratio, inferior because it has no way to give a
different balance to each zone. As you said it's turned off by default
so it had no tuning. The lower_zone_reserve_ratio has already been
tuned in 2.4. Somebody can attempt a conversion but it'll never be equal
since lower_zone_reserve_ratio is a superset of what
sysctl_lower_zone_protection can do.