POWER: Unexpected fault when writing to brk-allocated memory

[Nicholas Piggin]

On Sun, 5 Nov 2017 13:35:40 +0100
Florian Weimer <fweimer at redhat.com> wrote:

> On 11/05/2017 01:18 PM, Nicholas Piggin wrote:
> 
> > There was a recent change to move to 128TB address space by default,
> > and option for 512TB addresses if explicitly requested.  
> 
> Do you have a commit hash for the introduction of 128TB by default?  Thanks.

I guess this one

f6eedbba7a26 ("powerpc/mm/hash: Increase VA range to 128TB")

> 
> > Your brk request asked for > 128TB which the kernel gave it, but the
> > address limit in the paca that the SLB miss tests against was not
> > updated to reflect the switch to 512TB address space.
> > 
> > Why is your brk starting so high? Are you trying to test the > 128TB
> > case, or maybe something is confused by the 64->128TB change? What's
> > the strace look like if you run on a distro or <= 4.10 kernel?  
> 
> I think it is a consequence of running with an explicit loader 
> invocation.  With that, the heap is placed above ld.so, which can be 
> quite high in the address space.
> 
> I'm attaching two runs of cat, one executing directly as /bin/cat, and 
> one with /lib64/ld64.so.1 /bin/cat.
> 
> Fortunately, this does *not* apply to PIE binaries (also attached). 
> However, explicit loader invocations are sometimes used in test suites 
> (not just for glibc), and these sporadic test failures are quite annoying.
> 
> Do you still need the strace log?  And if yes, of what exactly?

Thanks, that should be quite helpful. I'll spend a bit more time to
study it, I'll let you know if I need any other traces.

> 
> > Something like the following patch may help if you could test.  
> 
> Okay, this will take some time.

It's no rush, there will probably be a revision to come.

Thanks,
Nick