Re: [RFC PATCH] mm: support CONFIG_ZONE_DEVICE + CONFIG_ZONE_DMA

[Joonsoo Kim]

On Tue, Jan 26, 2016 at 07:23:59PM -0800, Dan Williams wrote:
> On Tue, Jan 26, 2016 at 6:15 PM, Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:
> > On Tue, Jan 26, 2016 at 05:37:38PM -0800, Dan Williams wrote:
> >> On Tue, Jan 26, 2016 at 5:18 PM, Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:
> [..]
> >> > Please refer my previous attempt to add a new zone, ZONE_CMA.
> >> >
> >> > https://lkml.org/lkml/2015/2/12/84
> >> >
> >> > It salvages a bit from SECTION_WIDTH by increasing section size.
> >> > Similarly, I guess we can reduce NODE_WIDTH if needed although
> >> > it could cause to reduce maximum node size.
> >>
> >> Dave pointed out to me that LAST__PID_SHIFT might be a better
> >> candidate to reduce to 7 bits.  That field is for storing pids which
> >> are already bigger than 8 bits.  If it is relying on the fact that
> >> pids don't rollover very often then likely the impact of 7-bits
> >> instead of 8 will be minimal.
> >
> > Hmm... I'm not sure it's possible or not, but, it looks not a general
> > solution. It will solve your problem because you are using 64 bit arch
> > but other 32 bit archs can't get the benefit.
> 
> This is where the ZONE_CMA and ZONE_DEVICE efforts diverge.
> ZONE_DEVICE is meant to enable DMA access to hundreds of gigagbytes of
> persistent memory.  A 64-bit-only limitation for ZONE_DEVICE is
> reasonable.

Yes, but, my point is that if someone need another zone like as
ZONE_CMA, they couldn't get the benefit from this change. They need to
re-investigate what bits they can reduce and need to re-do all things.

If it is implemented more generally at this time, it can relieve their
burden and less churn the code. It would be helpful for maintainability.

Thanks.