netdev - Re: [PATCH v2 net-next 6/6] net: thunderbolt: Use kmap_local_page() instead of kmap

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Date:   Fri, 25 Nov 2022 16:41:10 +0100
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     netdev@...r.kernel.org,
        Anirudh Venkataramanan <anirudh.venkataramanan@...el.com>
Cc:     Ira Weiny <ira.weiny@...el.com>,
        Anirudh Venkataramanan <anirudh.venkataramanan@...el.com>,
        Michael Jamet <michael.jamet@...el.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Yehezkel Bernat <YehezkelShB@...il.com>
Subject: Re: [PATCH v2 net-next 6/6] net: thunderbolt: Use kmap_local_page() instead of kmap_atomic()

On mercoledì 23 novembre 2022 21:52:19 CET Anirudh Venkataramanan wrote:
> kmap_atomic() is being deprecated in favor of kmap_local_page(). Replace
> kmap_atomic() and kunmap_atomic() with kmap_local_page() and kunmap_local()
> respectively.
> 
> Note that kmap_atomic() disables preemption and page-fault processing, but
> kmap_local_page() doesn't. When converting uses of kmap_atomic(), one has
> to check if the code being executed between the map/unmap implicitly
> depends on page-faults and/or preemption being disabled. If yes, then code
> to disable page-faults and/or preemption should also be added for
> functional correctness. That however doesn't appear to be the case here,
> so just kmap_local_page() is used.
> 
> Also note that the page being mapped is not allocated by the driver, and so
> the driver doesn't know if the page is in normal memory. This is the reason
> kmap_local_page() is used as opposed to page_address().
> 
> I don't have hardware, so this change has only been compile tested.
> 
> Cc: Michael Jamet <michael.jamet@...el.com>
> Cc: Mika Westerberg <mika.westerberg@...ux.intel.com>
> Cc: Yehezkel Bernat <YehezkelShB@...il.com>
> Cc: Ira Weiny <ira.weiny@...el.com>
> Cc: Fabio M. De Francesco <fmdefrancesco@...il.com>
> Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@...el.com>
> ---
>  drivers/net/thunderbolt.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)

Reviewed-by: Fabio M. De Francesco <fmdefrancesco@...il.com>

Thanks,

Fabio

> diff --git a/drivers/net/thunderbolt.c b/drivers/net/thunderbolt.c
> index a52ee2b..b20cd37 100644
> --- a/drivers/net/thunderbolt.c
> +++ b/drivers/net/thunderbolt.c
> @@ -1051,7 +1051,7 @@ static void *tbnet_kmap_frag(struct sk_buff *skb,
> unsigned int frag_num, const skb_frag_t *frag =
> &skb_shinfo(skb)->frags[frag_num];
> 
>  	*len = skb_frag_size(frag);
> -	return kmap_atomic(skb_frag_page(frag)) + skb_frag_off(frag);
> +	return kmap_local_page(skb_frag_page(frag)) + skb_frag_off(frag);
>  }
> 
>  static netdev_tx_t tbnet_start_xmit(struct sk_buff *skb,
> @@ -1109,7 +1109,7 @@ static netdev_tx_t tbnet_start_xmit(struct sk_buff 
*skb,
> dest += len;
> 
>  			if (unmap) {
> -				kunmap_atomic(src);
> +				kunmap_local(src);
>  				unmap = false;
>  			}
> 
> @@ -1147,7 +1147,7 @@ static netdev_tx_t tbnet_start_xmit(struct sk_buff 
*skb,
> dest += len;
> 
>  		if (unmap) {
> -			kunmap_atomic(src);
> +			kunmap_local(src);
>  			unmap = false;
>  		}
> 
> @@ -1162,7 +1162,7 @@ static netdev_tx_t tbnet_start_xmit(struct sk_buff 
*skb,
> memcpy(dest, src, data_len);
> 
>  	if (unmap)
> -		kunmap_atomic(src);
> +		kunmap_local(src);
> 
>  	if (!tbnet_xmit_csum_and_map(net, skb, frames, frame_index + 1))
>  		goto err_drop;
> --
> 2.37.2