Subject: Re: [PATCH] net: Add back alignment for size for __alloc_skb

From: Eric Dumazet <[email-masked]>
Date: Wed, 02 Nov 2011 23:55:11 +0100

On 02/11/2011 23:43, Tony Lindgren wrote:

> Commit 87fb4b7b533073eeeaed0b6bf7c2328995f6c075 (net: more
> accurate skb truesize) changed the alignment of size. This
> can cause problems at least on some machines with NFS root:
> 
> Unhandled fault: alignment exception (0x801) at 0xc183a43a
> Internal error: : 801 [#1] PREEMPT
> Modules linked in:
> CPU: 0    Not tainted  (3.1.0-08784-g5eeee4a #733)
> pc : [<c02fbba0>]    lr : [<c02fbb9c>]    psr: 60000013
> sp : c180fef8  ip : 00000000  fp : c181f580
> r10: 00000000  r9 : c044b28c  r8 : 00000001
> r7 : c183a3a0  r6 : c1835be0  r5 : c183a412  r4 : 000001f2
> r3 : 00000000  r2 : 00000000  r1 : ffffffe6  r0 : c183a43a
> Flags: nZCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment kernel
> Control: 0005317f  Table: 10004000  DAC: 00000017
> Process swapper (pid: 1, stack limit = 0xc180e270)
> Stack: (0xc180fef8 to 0xc1810000)
> fee0:                                                       00000024 00000000
> ff00: 00000000 c183b9c0 c183b8e0 c044b28c c0507ccc c019dfc4 c180ff2c c0503cf8
> ff20: c180ff4c c180ff4c 00000000 c1835420 c182c740 c18349c0 c05233c0 00000000
> ff40: 00000000 c00e6bb8 c180e000 00000000 c04dd82c c0507e7c c050cc18 c183b9c0
> ff60: c05233c0 00000000 00000000 c01f34f4 c0430d70 c019d364 c04dd898 c04dd898
> ff80: c04dd82c c0507e7c c180e000 00000000 c04c584c c01f4918 c04dd898 c04dd82c
> ffa0: c04ddd28 c180e000 00000000 c0008758 c181fa60 3231d82c 00000037 00000000
> ffc0: 00000000 c04dd898 c04dd82c c04ddd28 00000013 00000000 00000000 00000000
> ffe0: 00000000 c04b2224 00000000 c04b21a0 c001056c c001056c 00000000 00000000
> Function entered at [<c02fbba0>] from [<c019dfc4>]
> Function entered at [<c019dfc4>] from [<c01f34f4>]
> Function entered at [<c01f34f4>] from [<c01f4918>]
> Function entered at [<c01f4918>] from [<c0008758>]
> Function entered at [<c0008758>] from [<c04b2224>]
> Function entered at [<c04b2224>] from [<c001056c>]
> Code: e1a00005 e3a01028 ebfa7cb0 e35a0000 (e5858028)
> 
> Here PC is at __alloc_skb and &shinfo->dataref is unaligned because
> skb->end can be unaligned without this patch.
> 
> Cc: Eric Dumazet <[email-masked]>
> Signed-off-by: Tony Lindgren <[email-masked]>
> 
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -189,6 +189,7 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
>  	 * aligned memory blocks, unless SLUB/SLAB debug is enabled.
>  	 * Both skb->head and skb_shared_info are cache line aligned.
>  	 */
> +	size = SKB_DATA_ALIGN(size);
>  	size += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
>  	data = kmalloc_node_track_caller(size, gfp_mask, node);
>  	if (!data)

Hmm, I dont think this is the right way to fix the bug.

There is a problem with your kmalloc() or alignments on your architecture.

What is the SMP_CACHE_BYTES value ?