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

From: "Fabio M. De Francesco" <[email-masked]>
Date: Fri, 25 Nov 2022 16:37:27 +0100

On mercoledì 23 novembre 2022 21:52:15 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: Ira Weiny <[email-masked]>
> Cc: Fabio M. De Francesco <[email-masked]>
> Cc: Edward Cree <[email-masked]>
> Cc: Martin Habets <[email-masked]>
> Signed-off-by: Anirudh Venkataramanan <[email-masked]>
> ---
> v1 -> v2: Update commit message
> ---
>  drivers/net/ethernet/sfc/tx.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 

Reviewed-by: Fabio M. De Francesco <[email-masked]>

Thanks,

Fabio

> 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