Subject: Re: [PATCH v2 net-next 3/6] cassini: Use page_address() instead of kmap_atomic()

From: "Fabio M. De Francesco" <[email-masked]>
Date: Fri, 25 Nov 2022 16:38:30 +0100

On mercoledì 23 novembre 2022 21:52:16 CET Anirudh Venkataramanan wrote:
> Pages for Rx buffers are allocated in cas_page_alloc() using either
> GFP_ATOMIC or GFP_KERNEL. Memory allocated with GFP_KERNEL/GFP_ATOMIC can't
> come from highmem and so there's no need to kmap() them. Just use
> page_address() instead. This makes the variable 'addr' unnecessary, so
> remove it too.
> 
> Note that kmap_atomic() disables preemption and page-fault processing,
> but page_address() doesn't. When removing 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
> page_address() is used.
> 
> 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]>
> Signed-off-by: Anirudh Venkataramanan <[email-masked]>
> ---
> v1 -> v2: Update commit message
> ---
>  drivers/net/ethernet/sun/cassini.c | 34 ++++++++++--------------------
>  1 file changed, 11 insertions(+), 23 deletions(-)

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

Thanks,

Fabio

> diff --git a/drivers/net/ethernet/sun/cassini.c
> b/drivers/net/ethernet/sun/cassini.c index 0aca193..2f66cfc 100644
> --- a/drivers/net/ethernet/sun/cassini.c
> +++ b/drivers/net/ethernet/sun/cassini.c
> @@ -1915,7 +1915,7 @@ static int cas_rx_process_pkt(struct cas *cp, struct
> cas_rx_comp *rxc, int off, swivel = RX_SWIVEL_OFF_VAL;
>  	struct cas_page *page;
>  	struct sk_buff *skb;
> -	void *addr, *crcaddr;
> +	void *crcaddr;
>  	__sum16 csum;
>  	char *p;
> 
> @@ -1936,7 +1936,7 @@ static int cas_rx_process_pkt(struct cas *cp, struct
> cas_rx_comp *rxc, skb_reserve(skb, swivel);
> 
>  	p = skb->data;
> -	addr = crcaddr = NULL;
> +	crcaddr = NULL;
>  	if (hlen) { /* always copy header pages */
>  		i = CAS_VAL(RX_COMP2_HDR_INDEX, words[1]);
>  		page = cp->rx_pages[CAS_VAL(RX_INDEX_RING, i)]
[CAS_VAL(RX_INDEX_NUM, i)];
> @@ -1948,12 +1948,10 @@ static int cas_rx_process_pkt(struct cas *cp, struct
> cas_rx_comp *rxc, i += cp->crc_size;
>  		dma_sync_single_for_cpu(&cp->pdev->dev, page->dma_addr + 
off,
>  					i, DMA_FROM_DEVICE);
> -		addr = cas_page_map(page->buffer);
> -		memcpy(p, addr + off, i);
> +		memcpy(p, page_address(page->buffer) + off, i);
>  		dma_sync_single_for_device(&cp->pdev->dev,
>  					   page->dma_addr + off, i,
>  					   DMA_FROM_DEVICE);
> -		cas_page_unmap(addr);
>  		RX_USED_ADD(page, 0x100);
>  		p += hlen;
>  		swivel = 0;
> @@ -1984,12 +1982,11 @@ static int cas_rx_process_pkt(struct cas *cp, struct
> cas_rx_comp *rxc, /* make sure we always copy a header */
>  		swivel = 0;
>  		if (p == (char *) skb->data) { /* not split */
> -			addr = cas_page_map(page->buffer);
> -			memcpy(p, addr + off, RX_COPY_MIN);
> +			memcpy(p, page_address(page->buffer) + off,
> +			       RX_COPY_MIN);
>  			dma_sync_single_for_device(&cp->pdev->dev,
>  						   page->dma_addr 
+ off, i,
DMA_FROM_DEVICE);
> -			cas_page_unmap(addr);
>  			off += RX_COPY_MIN;
>  			swivel = RX_COPY_MIN;
>  			RX_USED_ADD(page, cp->mtu_stride);
> @@ -2036,10 +2033,8 @@ static int cas_rx_process_pkt(struct cas *cp, struct
> cas_rx_comp *rxc, RX_USED_ADD(page, hlen + cp->crc_size);
>  		}
> 
> -		if (cp->crc_size) {
> -			addr = cas_page_map(page->buffer);
> -			crcaddr  = addr + off + hlen;
> -		}
> +		if (cp->crc_size)
> +			crcaddr = page_address(page->buffer) + off + 
hlen;
> 
>  	} else {
>  		/* copying packet */
> @@ -2061,12 +2056,10 @@ static int cas_rx_process_pkt(struct cas *cp, struct
> cas_rx_comp *rxc, i += cp->crc_size;
>  		dma_sync_single_for_cpu(&cp->pdev->dev, page->dma_addr + 
off,
>  					i, DMA_FROM_DEVICE);
> -		addr = cas_page_map(page->buffer);
> -		memcpy(p, addr + off, i);
> +		memcpy(p, page_address(page->buffer) + off, i);
>  		dma_sync_single_for_device(&cp->pdev->dev,
>  					   page->dma_addr + off, i,
>  					   DMA_FROM_DEVICE);
> -		cas_page_unmap(addr);
>  		if (p == (char *) skb->data) /* not split */
>  			RX_USED_ADD(page, cp->mtu_stride);
>  		else
> @@ -2081,20 +2074,17 @@ static int cas_rx_process_pkt(struct cas *cp, struct
> cas_rx_comp *rxc, page->dma_addr,
>  						dlen + cp-
>crc_size,
>  						DMA_FROM_DEVICE);
> -			addr = cas_page_map(page->buffer);
> -			memcpy(p, addr, dlen + cp->crc_size);
> +			memcpy(p, page_address(page->buffer), dlen + cp-
>crc_size);
>  			dma_sync_single_for_device(&cp->pdev->dev,
>  						   page->dma_addr,
>  						   dlen + cp-
>crc_size,
>  						   
DMA_FROM_DEVICE);
> -			cas_page_unmap(addr);
>  			RX_USED_ADD(page, dlen + cp->crc_size);
>  		}
>  end_copy_pkt:
> -		if (cp->crc_size) {
> -			addr    = NULL;
> +		if (cp->crc_size)
>  			crcaddr = skb->data + alloclen;
> -		}
> +
>  		skb_put(skb, alloclen);
>  	}
> 
> @@ -2103,8 +2093,6 @@ static int cas_rx_process_pkt(struct cas *cp, struct
> cas_rx_comp *rxc, /* checksum includes FCS. strip it out. */
>  		csum = csum_fold(csum_partial(crcaddr, cp->crc_size,
>  					      csum_unfold(csum)));
> -		if (addr)
> -			cas_page_unmap(addr);
>  	}
>  	skb->protocol = eth_type_trans(skb, cp->dev);
>  	if (skb->protocol == htons(ETH_P_IP)) {
> --
> 2.37.2