Subject: Re: [PATCH net-next 2/5] sfc: Use kmap_local_page() instead of kmap_atomic()

From: "Fabio M. De Francesco" <[email-masked]>
Date: Fri, 18 Nov 2022 09:23:31 +0100

On giovedì 17 novembre 2022 23:25:54 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. Converting the former to the latter is safe only
> if there isn't an implicit dependency on preemption and page-fault handling
> being disabled, which does appear to be the case here.

NIT: It is always possible to disable explicitly along with the conversion.
However, you are noticing that we don't need to do it.

I'm noticing the same wording in other of your patches, but there are no 
problems with them. 

> 
> 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]>
> ---
>  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

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

Thanks,

Fabio