Re: 4.0.0-rc4: panic in free_block

[David Ahern]

On 3/20/15 4:49 PM, David Ahern wrote:
> On 3/20/15 3:17 PM, Linus Torvalds wrote:
> > In other words, if I read that sparc asm right (and it is very likely
> > that I do *not*), then "objp" is NULL, and that's why you crash.
>
> That does appear to be why. I put a WARN_ON before
> clear_obj_pfmemalloc() if objpp[i] is NULL. I got 2 splats during an
> 'allyesconfig' build and the system stayed up.
>
> > That's odd, because we know that objp cannot be NULL in
> > kmem_slab_free() (even if we allowed it, like with kfree(),
> > remove_vma() cannot possibly have a NULL vma, since ti dereferences it
> > multiple times).
> >
> > So I must be misreading this completely. Somebody with better sparc
> > debugging mojo should double-check my logic. How would objp be NULL?
>
> I'll add checks to higher layers and see if it reveals anything.
>
> I did ask around and apparently this bug is hit only with the new M7
> processors. DaveM: that's why you are not hitting this.

Here's another data point: If I disable NUMA I don't see the problem. 
Performance drops, but no NULL pointer splats which would have been panics.

The 128 cpu ldom with NUMA enabled shows the problem every single time I 
do a kernel compile (-j 128). With NUMA disabled I have done 3 
allyesconfig compiles without hitting the problem. I'll put the compiles 
into a loop while I head out for dinner.

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