Problems with THP in v4.5-rc4 on POWER

[Paul Mackerras]

On Sat, Feb 20, 2016 at 12:39:42PM +1100, Paul Mackerras wrote:
> It seems there's something wrong with our transparent hugepage
> implementation on POWER server processors as of v4.5-rc4.  I have seen
> the email thread on "[BUG] random kernel crashes after THP rework on
> s390 (maybe also on PowerPC and ARM)", but this doesn't seem exactly
> the same as that (though it may of course be related).
> 
> I have been testing v4.5-rc4 with Aneesh's patch "powerpc/mm/hash:
> Clear the invalid slot information correctly" on top, on a KVM guest
> with 160 vcpus (threads=8) and 32GB of memory backed by 16MB large
> pages, running on a POWER8 machine running a 4.4.1 host kernel (20
> cores * 8 threads, 128GB of RAM).  The guest kernel is compiled with
> THP enabled and set to "always" (i.e. not "madvise").
> 
> On this setup, when doing something like a large kernel compile, I see
> random segfaults happening (in gcc, cc1, sh, etc.).  I also see bursts
> of messages like this on the host console:
> 
> [50957.570859] Harmless Hypervisor Maintenance interrupt [Recovered]
> [50957.570864]  Error detail: Processor Recovery done
> [50957.570869]  HMER: 2040000000000000

When I use a merge of v4.5-rc4 with the fixes branch from the powerpc
tree, I don't see these messages any more, presumably due to
"powerpc/mm: Fix Multi hit ERAT cause by recent THP update".  With my
patch, I still see that it is finding HPTEs to invalidate, but without
my patch, even though it is presumably leaving HPTEs around, I don't
see any errors (such as random segfaults) occurring.

Paul.