netdev - Re: [PATCH net-next 4/4] net: add dedicated kmem

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Message-ID: <20230203114744.153c4063@kernel.org>
Date:   Fri, 3 Feb 2023 11:47:44 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Paolo Abeni <pabeni@...hat.com>,
        "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        eric.dumazet@...il.com, Alexander Duyck <alexanderduyck@...com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>
Subject: Re: [PATCH net-next 4/4] net: add dedicated kmem_cache for
 typical/small skb->head

On Fri, 3 Feb 2023 09:17:55 +0100 Eric Dumazet wrote:
> > I *think* non power of two size is also required to avoid an issue
> > plain (no GFP_DMA nor __GFP_ACCOUNT) allocations in case of fallback to
> > kmalloc(), to prevent skb_kfree_head() mis-interpreting skb->head as
> > kmem_cache allocated.  
> 
> Indeed there are multiple cases explaining why SKB_SMALL_HEAD_CACHE_SIZE
> needs to be a non power of two.

Since you may need to respin would you mind spelling this out a bit
more in the comment ? Maybe: 

-/* We want SKB_SMALL_HEAD_CACHE_SIZE to not be a power of two. */
+/* We want SKB_SMALL_HEAD_CACHE_SIZE to not be a power of two.
+ * This should ensure that SKB_SMALL_HEAD_HEADROOM is a unique
+ * size, and we can differentiate heads from skb_small_head_cache
+ * vs system slabs by looking at their size (skb_end_offset()).
+ */