powerpc/mm: Only define MAX_PHYSMEM_BITS in SPARSEMEM configurations

[Christophe Leroy]

Le 25/03/2019 à 12:35, Michael Ellerman a écrit :
> Ben Hutchings <ben at decadent.org.uk> writes:
>> On Mon, 2019-03-25 at 01:03 +0100, Andreas Schwab wrote:
>>> On Mär 24 2019, Ben Hutchings <ben at decadent.org.uk> wrote:
>>>
>>>> Presumably you have CONFIG_PPC_BOOK3S_64 enabled and
>>>> CONFIG_SPARSEMEM
>>>> disabled?  Was this configuration actually usable?
>>>
>>> Why not?
>>
>> I assume that CONFIG_SPARSEMEM is the default for a good reason.
>> What I don't know is how strong that reason is (I am not a Power expert
>> at all).  Looking a bit further, it seems to be related to CONFIG_NUMA
>> in that you can enable CONFIG_FLATMEM if and only if that's disabled.
>> So I suppose the configuration you used works for non-NUMA systems.
> 
> Aneesh pointed out this fix would break FLATMEM after I'd merged it, but
> it didn't break any of our defconfigs so I wondered if anyone would
> notice.
> 
> I checked today and a G5 will boot with FLATMEM, which I assume is what
> Andreas is using.
> 
> I guess we should fix this build break for now.
> 
> Even some G5's have discontiguous memory, so FLATMEM is not clearly a
> good choice even for all G5's, and actually a fresh g5_defconfig uses
> SPARSEMEM.
> 
> So I'm inclined to just switch to always using SPARSEMEM on 64-bit
> Book3S, because that's what's well tested and we hardly need more code
> paths to test. Unless anyone has a strong objection, I haven't actually
> benchmarked FLATMEM vs SPARSEMEM on a G5.
> 

Christian Zigotzky reported a build failure with this patch.

Christophe