[3/3,net-next] net_device: Add minimal padding to allow for net_device pointer alignment

Makes use of the __alignof__ GCC builtin to determine the minimal
amount of padding required to align the pointer afterwards.

Due to use of ____cacheline_aligned_in_smp inside net_device,
struct net_device is actually always a multiple of SMP_CACHE_BYTES
so typically no padding is needed but this logic is kept in place
in case that is no longer the case.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
---
 include/linux/kernel.h | 2 ++
 net/core/dev.c         | 3 +--
 2 files changed, 3 insertions(+), 2 deletions(-)

On Fri, 2015-04-10 at 01:43 +0200, Thomas Graf wrote:
> Makes use of the __alignof__ GCC builtin to determine the minimal
> amount of padding required to align the pointer afterwards.
> 
> Due to use of ____cacheline_aligned_in_smp inside net_device,
> struct net_device is actually always a multiple of SMP_CACHE_BYTES
> so typically no padding is needed but this logic is kept in place
> in case that is no longer the case.
> 
> Signed-off-by: Thomas Graf <tgraf@suug.ch>
> ---
>  include/linux/kernel.h | 2 ++
>  net/core/dev.c         | 3 +--
>  2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index d6d630d..4ec018f 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -50,6 +50,8 @@
>  #define __ALIGN_MASK(x, mask)	__ALIGN_KERNEL_MASK((x), (mask))
>  #define PTR_ALIGN(p, a)		((typeof(p))ALIGN((unsigned long)(p), (a)))
>  #define IS_ALIGNED(x, a)		(((x) & ((typeof(x))(a) - 1)) == 0)
> +#define PTR_ALIGN_PAD(type, a) \
> +	({ (__alignof__(type) < (a)) ? (a) - __alignof__(type) : 0; })
>  
>  #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) + __must_be_array(arr))
>  
> diff --git a/net/core/dev.c b/net/core/dev.c
> index b2775f0..2b43aba 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -6794,8 +6794,7 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
>  		alloc_size = ALIGN(alloc_size, NETDEV_ALIGN);
>  		alloc_size += sizeof_priv;
>  	}
> -	/* ensure 32-byte alignment of whole construct */
> -	alloc_size += NETDEV_ALIGN - 1;
> +	alloc_size += PTR_ALIGN_PAD(struct net_device, NETDEV_ALIGN);
>  
>  	p = kzalloc(alloc_size, GFP_KERNEL | __GFP_NOWARN | __GFP_REPEAT);
>  	if (!p)


I believe code intent was to get an alignment even if kmalloc() returns
say 0xxxxx0008.

AFAIK, SLAB in debug mode was able to do this.

On 04/09/15 at 05:34pm, Eric Dumazet wrote:
> On Fri, 2015-04-10 at 01:43 +0200, Thomas Graf wrote:
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index b2775f0..2b43aba 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -6794,8 +6794,7 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
> >  		alloc_size = ALIGN(alloc_size, NETDEV_ALIGN);
> >  		alloc_size += sizeof_priv;
> >  	}
> > -	/* ensure 32-byte alignment of whole construct */
> > -	alloc_size += NETDEV_ALIGN - 1;
> > +	alloc_size += PTR_ALIGN_PAD(struct net_device, NETDEV_ALIGN);
> >  
> >  	p = kzalloc(alloc_size, GFP_KERNEL | __GFP_NOWARN | __GFP_REPEAT);
> >  	if (!p)
> 
> 
> I believe code intent was to get an alignment even if kmalloc() returns
> say 0xxxxx0008.
> 
> AFAIK, SLAB in debug mode was able to do this.

I assumed kmalloc would guarantee alignment of the return pointer as
indicated by __alignof__ but I now realize that this can't possibly be the
case. kmalloc only knows about the size. We'll have to drop patch 3.
Fortunately it's still below 2K.

On Fri, 2015-04-10 at 09:48 +0100, Thomas Graf wrote:

> 
> I assumed kmalloc would guarantee alignment of the return pointer as
> indicated by __alignof__ but I now realize that this can't possibly be the
> case. kmalloc only knows about the size. We'll have to drop patch 3.
> Fortunately it's still below 2K.


If you really care for this, take a look at qdisc_alloc()
and commit d276055c4e90a7278cd5167ba9755c9b214bcff7
("net_sched: reduce fifo qdisc size")

(Note that some setups require ~3 network devices but 3000+ qdisc)

I guess it might be nice to define a common helper and reuse it.