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

[Paolo Abeni <pabeni@redhat.com>]

On Thu, 2023-02-02 at 05:20 +0000, Michael Kelley (LINUX) wrote:
> From: Jakub Kicinski <kuba@kernel.org> 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.

Ok

> 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?

While at that, please also have a look at the following allocation in
netvsc_dma_map():

	packet->dma_range = kcalloc(page_count,
                                    sizeof(*packet->dma_range),
                                    GFP_KERNEL);

which looks wrong - netvsc_dma_map() should be in atomic context.

Anyway it's a topic unrelated from this patch. I just stumbled upon it
while reviewing.

Cheers,

Paolo