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net: add skbuff_clear() helper

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clang is unable to inline the memset() calls in net/core/skbuff.c
when initializing allocated sk_buff.

memset(skb, 0, offsetof(struct sk_buff, tail));

This is unfortunate, because:

1) calling external memset_orig() helper adds a call/ret and
   typical setup cost.

2) offsetof(struct sk_buff, tail) == 0xb8 = 0x80 + 0x38

   On x86_64, memset_orig() performs two 64 bytes clear,
   then has to loop 7 times to clear the final 56 bytes.

skbuff_clear() makes sure the minimal and optimal code
is generated.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260109203836.1667441-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Eric Dumazet 2026-01-09 20:38:36 +00:00 committed by Jakub Kicinski
parent d7161b0790
commit 0391ab577c
1 changed files with 22 additions and 10 deletions
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32
net/core/skbuff.c
View File

@ -307,6 +307,23 @@ static struct sk_buff *napi_skb_cache_get(bool alloc)
return skb;
}
/*
* Only clear those fields we need to clear, not those that we will
* actually initialise later. Hence, don't put any more fields after
* the tail pointer in struct sk_buff!
*/
static inline void skbuff_clear(struct sk_buff *skb)
{
/* Replace memset(skb, 0, offsetof(struct sk_buff, tail))
* with two smaller memset(), with a barrier() between them.
* This forces the compiler to inline both calls.
*/
BUILD_BUG_ON(offsetof(struct sk_buff, tail) <= 128);
memset(skb, 0, 128);
barrier();
memset((void *)skb + 128, 0, offsetof(struct sk_buff, tail) - 128);
}
/**
* napi_skb_cache_get_bulk - obtain a number of zeroed skb heads from the cache
* @skbs: pointer to an at least @n-sized array to fill with skb pointers
@ -357,7 +374,7 @@ u32 napi_skb_cache_get_bulk(void **skbs, u32 n)
skbs[i] = nc->skb_cache[base + i];
kasan_mempool_unpoison_object(skbs[i], skbuff_cache_size);
memset(skbs[i], 0, offsetof(struct sk_buff, tail));
skbuff_clear(skbs[i]);
}
nc->skb_count -= n;
@ -424,7 +441,7 @@ struct sk_buff *slab_build_skb(void *data)
if (unlikely(!skb))
return NULL;
memset(skb, 0, offsetof(struct sk_buff, tail));
skbuff_clear(skb);
data = __slab_build_skb(data, &size);
__finalize_skb_around(skb, data, size);
@ -476,7 +493,7 @@ struct sk_buff *__build_skb(void *data, unsigned int frag_size)
if (unlikely(!skb))
return NULL;
memset(skb, 0, offsetof(struct sk_buff, tail));
skbuff_clear(skb);
__build_skb_around(skb, data, frag_size);
return skb;
@ -537,7 +554,7 @@ static struct sk_buff *__napi_build_skb(void *data, unsigned int frag_size)
if (unlikely(!skb))
return NULL;
memset(skb, 0, offsetof(struct sk_buff, tail));
skbuff_clear(skb);
__build_skb_around(skb, data, frag_size);
return skb;
@ -696,12 +713,7 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
*/
prefetchw(data + SKB_WITH_OVERHEAD(size));
/*
* Only clear those fields we need to clear, not those that we will
* actually initialise below. Hence, don't put any more fields after
* the tail pointer in struct sk_buff!
*/
memset(skb, 0, offsetof(struct sk_buff, tail));
skbuff_clear(skb);
__build_skb_around(skb, data, size);
skb->pfmemalloc = pfmemalloc;