linux-kernel - Re: unaligned accesses in SLAB etc.

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Message-Id: <20141012.132012.254712930139255731.davem@davemloft.net>
Date:	Sun, 12 Oct 2014 13:20:12 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	linux-kernel@...r.kernel.org
Cc:	cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
	iamjoonsoo.kim@....com, akpm@...ux-foundation.org,
	linux-mm@...ck.org, mroos@...ux.ee, sparclinux@...r.kernel.org
Subject: Re: unaligned accesses in SLAB etc.

From: David Miller <davem@...emloft.net>
Date: Sat, 11 Oct 2014 22:15:10 -0400 (EDT)

> 
> I'm getting tons of the following on sparc64:
> 
> [603965.383447] Kernel unaligned access at TPC[546b58] free_block+0x98/0x1a0
> [603965.396987] Kernel unaligned access at TPC[546b60] free_block+0xa0/0x1a0
> [603965.410523] Kernel unaligned access at TPC[546b58] free_block+0x98/0x1a0

The unaligned accesses are happening in the SLAB_OBJ_PFMEMALLOC code,
which assumes that all object pointers are "unsigned long" aligned:

static inline void set_obj_pfmemalloc(void **objp)
{
        *objp = (void *)((unsigned long)*objp | SLAB_OBJ_PFMEMALLOC);
        return;
}

etc. etc.

But that code has been there working forever.  Something changed
recently such that this assumption no longer holds.

In all of the cases, the address is 4-byte aligned but not 8-byte
aligned.  And they are vmalloc addresses.

Which made me suspect the percpu commit:

====================
commit bf0dea23a9c094ae869a88bb694fbe966671bf6d
Author: Joonsoo Kim <iamjoonsoo.kim@....com>
Date:   Thu Oct 9 15:26:27 2014 -0700

    mm/slab: use percpu allocator for cpu cache
====================

And indeed, reverting this commit fixes the problem.
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