linux-kernel - Re: 4.0.0-rc4: panic in free

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Message-ID: <550CA3F9.9040201@oracle.com>
Date:	Fri, 20 Mar 2015 16:49:29 -0600
From:	David Ahern <david.ahern@...cle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	linux-mm <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: 4.0.0-rc4: panic in free_block

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.

David

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