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

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

On Tue, Jan 26, 2016 at 08:26:24PM -0800, Dan Williams wrote:
> On Tue, Jan 26, 2016 at 7:52 PM, Joonsoo Kim <iamjoonsoo.kim@lge.com> wrote:
> > 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@lge.com> 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@lge.com> 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.
> 
> I agree in principle that finding a 32-bit compatible solution is
> desirable, but it simply may not be feasible.

Okay.

> 
> For now, I'll help with auditing the existing bits so we can enumerate
> the tradeoffs.

Thanks! :)

> Hmm, one tradeoff that comes to mind for 32-bit is sacrificing
> ZONE_HIGHMEM, for ZONE_CMA.  Are there configurations that need both
> enabled?  If a platform needs highmem it really should be using a
> 64-bit kernel (if possible), desire for ZONE_CMA might be a nice
> encouragement to lessen the prevalence of highmem.

I guess that it's not possible. There are many systems that need
both.

I don't think deeply, but, there is another option for ZONE_CMA.
It can share ZONE_MOVABLE because their chracteristic is roughly
same in view of MM. I will think more.

Thanks.

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