[PATCH 2/5] powerpc/64s/hash: Allow MAP_FIXED allocations to cross 128TB boundary

[Michael Ellerman]

Nicholas Piggin <npiggin at gmail.com> writes:

> On Mon, 06 Nov 2017 16:14:10 +0530
> "Aneesh Kumar K.V" <aneesh.kumar at linux.vnet.ibm.com> wrote:
>
>> Nicholas Piggin <npiggin at gmail.com> writes:
>> 
>> > While mapping hints with a length that cross 128TB are disallowed,
>> > MAP_FIXED allocations that cross 128TB are allowed. These are failing
>> > on hash (on radix they succeed). Add an additional case for fixed
>> > mappings to expand the addr_limit when crossing 128TB.  
>> 
>> Shouldn't that be fixed in radix. But i see x86 also doing this?
>> 
>> 
>> 	if (flags & MAP_FIXED)
>> 		return addr;
>> 
>> Kiril,
>> 
>> Is that expected?
>
> I should actually reply to this one because the other did not
> have Kirill on cc.
>
> Generic mapping code appears it will always succeed when given an
> explicit hint request, even if the address is below the boundary
> and address + length is above it. Even when !MAP_FIXED. This is the
> sane behaviour AFAIKS.

It's "sane" if you want the 128T boundary to be invisible.

But the whole goddamn point was that we wanted apps to have to opt-in to
using > 128T, and having brk accidentally go over the 128T boundary does
not count as opting in.

So it's a mess as usual.

> So we should switch powerpc to match, shouldn't we?

Yes we should do whatever the other arches do. 

Actually we should do what the man page describes .. except it doesn't.

cheers