netdev - Re: [PATCH v2] net: allow skb_datagram_iter to be called from any context

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Date: Sun, 23 Jun 2024 16:44:34 +0300
From: Sagi Grimberg <sagi@...mberg.me>
To: netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>
Cc: Eric Dumazet <edumazet@...gle.com>, David Howells <dhowells@...hat.com>,
 Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v2] net: allow skb_datagram_iter to be called from any
 context



On 23/06/2024 11:12, Sagi Grimberg wrote:
> We only use the mapping in a single context, so kmap_local is sufficient
> and cheaper. Make sure to use skb_frag_foreach_page as skb frags may
> contain highmem compound pages and we need to map page by page.
>
> Signed-off-by: Sagi Grimberg <sagi@...mberg.me>
> ---
> Changes from v2:

Changes from v1 actually...

> - Fix usercopy BUG() due to copy from highmem pages across page boundary
>    by using skb_frag_foreach_page
>
>   net/core/datagram.c | 19 +++++++++++++------
>   1 file changed, 13 insertions(+), 6 deletions(-)
>
> diff --git a/net/core/datagram.c b/net/core/datagram.c
> index a8b625abe242..cb72923acc21 100644
> --- a/net/core/datagram.c
> +++ b/net/core/datagram.c
> @@ -435,15 +435,22 @@ static int __skb_datagram_iter(const struct sk_buff *skb, int offset,
>   
>   		end = start + skb_frag_size(frag);
>   		if ((copy = end - offset) > 0) {
> -			struct page *page = skb_frag_page(frag);
> -			u8 *vaddr = kmap(page);
> +			u32 p_off, p_len, copied;
> +			struct page *p;
> +			u8 *vaddr;
>   
>   			if (copy > len)
>   				copy = len;
> -			n = INDIRECT_CALL_1(cb, simple_copy_to_iter,
> -					vaddr + skb_frag_off(frag) + offset - start,
> -					copy, data, to);
> -			kunmap(page);
> +
> +			skb_frag_foreach_page(frag,
> +					      skb_frag_off(frag) + offset - start,
> +					      copy, p, p_off, p_len, copied) {
> +				vaddr = kmap_local_page(p);
> +				n = INDIRECT_CALL_1(cb, simple_copy_to_iter,
> +					vaddr + p_off, p_len, data, to);
> +				kunmap_local(vaddr);
> +			}
> +
>   			offset += n;
>   			if (n != copy)
>   				goto short_copy;