RE: [PATCH net 1/1] hv_netvsc: Fix missed pagebuf entries in netvsc_dma_map/unmap()

[Michael Kelley (LINUX)]

From: Jakub Kicinski <kuba@xxxxxxxxxx> Sent: Wednesday, February 1, 2023 9:01 PM
> 
> On Mon, 30 Jan 2023 19:33:06 -0800 Michael Kelley wrote:
> > @@ -990,9 +987,7 @@ static int netvsc_dma_map(struct hv_device *hv_dev,
> >  			  struct hv_netvsc_packet *packet,
> >  			  struct hv_page_buffer *pb)
> >  {
> > -	u32 page_count =  packet->cp_partial ?
> > -		packet->page_buf_cnt - packet->rmsg_pgcnt :
> > -		packet->page_buf_cnt;
> > +	u32 page_count = packet->page_buf_cnt;
> >  	dma_addr_t dma;
> >  	int i;
> 
> Suspiciously, the caller still does:
> 
>                 if (packet->cp_partial)
>                         pb += packet->rmsg_pgcnt;
> 
>                 ret = netvsc_dma_map(ndev_ctx->device_ctx, packet, pb);
> 
> Shouldn't that if () pb +=... also go away?

No -- it's correct.

In netvsc_send(), cp_partial is tested and packet->page_buf_cnt is
adjusted.  But the pointer into the pagebuf array is not adjusted in
netvsc_send().  Instead it is adjusted here in netvsc_send_pkt(), which
brings it back in sync with packet->page_buf_cnt.

I don't know if there's a good reason for the adjustment being split
across two different functions.  It doesn't seem like the most
straightforward approach.  From a quick glance at the code it looks
like this adjustment to 'pb' could move to netvsc_send() to be
together with the adjustment to packet->page_buf_cnt,  but maybe
there's a reason for the split that I'm not familiar with.

Haiyang -- any insight?

Michael