netdev - Re: [PATCH v2 net-next 2/6] sfc: Use kmap_local_page() instead of kmap

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Message-ID: <Y38skj5oq2IYUB2D@gmail.com>
Date:   Thu, 24 Nov 2022 08:34:26 +0000
From:   Martin Habets <habetsm.xilinx@...il.com>
To:     Anirudh Venkataramanan <anirudh.venkataramanan@...el.com>
Cc:     netdev@...r.kernel.org, Ira Weiny <ira.weiny@...el.com>,
        "Fabio M . De Francesco" <fmdefrancesco@...il.com>,
        Edward Cree <ecree.xilinx@...il.com>
Subject: Re: [PATCH v2 net-next 2/6] sfc: Use kmap_local_page() instead of
 kmap_atomic()

On Wed, Nov 23, 2022 at 12:52:15PM -0800, 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: Ira Weiny <ira.weiny@...el.com>
> Cc: Fabio M. De Francesco <fmdefrancesco@...il.com>
> Cc: Edward Cree <ecree.xilinx@...il.com>
> Cc: Martin Habets <habetsm.xilinx@...il.com>
> Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@...el.com>

Acked-by: Martin Habets <habetsm.xilinx@...il.com>

> ---
> v1 -> v2: Update commit message
> ---
>  drivers/net/ethernet/sfc/tx.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/sfc/tx.c b/drivers/net/ethernet/sfc/tx.c
> index c5f88f7..4ed4082 100644
> --- a/drivers/net/ethernet/sfc/tx.c
> +++ b/drivers/net/ethernet/sfc/tx.c
> @@ -207,11 +207,11 @@ static void efx_skb_copy_bits_to_pio(struct efx_nic *efx, struct sk_buff *skb,
>  		skb_frag_t *f = &skb_shinfo(skb)->frags[i];
>  		u8 *vaddr;
>  
> -		vaddr = kmap_atomic(skb_frag_page(f));
> +		vaddr = kmap_local_page(skb_frag_page(f));
>  
>  		efx_memcpy_toio_aligned_cb(efx, piobuf, vaddr + skb_frag_off(f),
>  					   skb_frag_size(f), copy_buf);
> -		kunmap_atomic(vaddr);
> +		kunmap_local(vaddr);
>  	}
>  
>  	EFX_WARN_ON_ONCE_PARANOID(skb_shinfo(skb)->frag_list);
> -- 
> 2.37.2