linux-kernel - Re: [PATCH net-next 01/17] net: Copy slab data for sendmsg(MSG_SPLICE

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Message-ID: <ZIy82RjpmQMTNafS@corigine.com>
Date:   Fri, 16 Jun 2023 21:49:45 +0200
From:   Simon Horman <simon.horman@...igine.com>
To:     David Howells <dhowells@...hat.com>
Cc:     netdev@...r.kernel.org,
        Alexander Duyck <alexander.duyck@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        David Ahern <dsahern@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Menglong Dong <imagedong@...cent.com>
Subject: Re: [PATCH net-next 01/17] net: Copy slab data for
 sendmsg(MSG_SPLICE_PAGES)

On Fri, Jun 16, 2023 at 05:12:44PM +0100, David Howells wrote:

...

> +/**
> + * alloc_skb_frag - Allocate a page fragment for using in a socket
> + * @fragsz: The size of fragment required
> + * @gfp: Allocation flags
> + */
> +void *alloc_skb_frag(size_t fragsz, gfp_t gfp)
> +{
> +	struct skb_splice_frag_cache *cache;
> +	struct folio *folio, *spare = NULL;
> +	size_t offset, fsize;
> +	void *p;
> +
> +	if (WARN_ON_ONCE(fragsz == 0))
> +		fragsz = 1;
> +
> +	cache = get_cpu_ptr(&skb_splice_frag_cache);
> +reload:
> +	folio = cache->folio;
> +	offset = cache->offset;
> +try_again:
> +	if (fragsz > offset)
> +		goto insufficient_space;
> +
> +	/* Make the allocation. */
> +	cache->pagecnt_bias--;
> +	offset = ALIGN_DOWN(offset - fragsz, SMP_CACHE_BYTES);
> +	cache->offset = offset;
> +	p = cache->virt + offset;
> +	put_cpu_ptr(skb_splice_frag_cache);

Hi David,

I don't think it makes any difference at run-time.
But to keep Sparse happy, perhaps this ought to be put_cpu_var()

...