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Date: Thu, 09 Apr 2015 17:34:55 -0700 From: Eric Dumazet <eric.dumazet@...il.com> To: Thomas Graf <tgraf@...g.ch> Cc: davem@...emloft.net, jeffrey.t.kirsher@...el.com, intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org Subject: Re: [PATCH 3/3 net-next] net_device: Add minimal padding to allow for net_device pointer alignment 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@...g.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. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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