Re: [PATCH v2] net: ethernet: mtk_eth_soc: fix misuse of mem alloc interface netdev[napi]_alloc_frag

[Alexander Duyck]

On Fri, Jun 3, 2022 at 8:25 AM Alexander Duyck
<alexander.duyck@xxxxxxxxx> wrote:
>
> On Fri, Jun 3, 2022 at 2:03 AM Chen Lin <chen45464546@xxxxxxx> wrote:
> >
> > When rx_flag == MTK_RX_FLAGS_HWLRO,
> > rx_data_len = MTK_MAX_LRO_RX_LENGTH(4096 * 3) > PAGE_SIZE.
> > netdev_alloc_frag is for alloction of page fragment only.
> > Reference to other drivers and Documentation/vm/page_frags.rst
> >
> > Branch to use alloc_pages when ring->frag_size > PAGE_SIZE.
> >
> > Signed-off-by: Chen Lin <chen45464546@xxxxxxx>
> > ---
> >  drivers/net/ethernet/mediatek/mtk_eth_soc.c |   22 ++++++++++++++++++++--
> >  1 file changed, 20 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/net/ethernet/mediatek/mtk_eth_soc.c b/drivers/net/ethernet/mediatek/mtk_eth_soc.c
> > index b3b3c07..772d903 100644
> > --- a/drivers/net/ethernet/mediatek/mtk_eth_soc.c
> > +++ b/drivers/net/ethernet/mediatek/mtk_eth_soc.c
> > @@ -1467,7 +1467,16 @@ static int mtk_poll_rx(struct napi_struct *napi, int budget,
> >                         goto release_desc;
> >
> >                 /* alloc new buffer */
> > -               new_data = napi_alloc_frag(ring->frag_size);
> > +               if (ring->frag_size <= PAGE_SIZE) {
> > +                       new_data = napi_alloc_frag(ring->frag_size);
> > +               } else {
> > +                       struct page *page;
> > +                       unsigned int order = get_order(ring->frag_size);
> > +
> > +                       page = alloc_pages(GFP_ATOMIC | __GFP_COMP |
> > +                                           __GFP_NOWARN, order);
> > +                       new_data = page ? page_address(page) : NULL;
> > +               }
> >                 if (unlikely(!new_data)) {
> >                         netdev->stats.rx_dropped++;
> >                         goto release_desc;
> > @@ -1914,7 +1923,16 @@ static int mtk_rx_alloc(struct mtk_eth *eth, int ring_no, int rx_flag)
> >                 return -ENOMEM;
> >
> >         for (i = 0; i < rx_dma_size; i++) {
> > -               ring->data[i] = netdev_alloc_frag(ring->frag_size);
> > +               if (ring->frag_size <= PAGE_SIZE) {
> > +                       ring->data[i] = netdev_alloc_frag(ring->frag_size);
> > +               } else {
> > +                       struct page *page;
> > +                       unsigned int order = get_order(ring->frag_size);
> > +
> > +                       page = alloc_pages(GFP_KERNEL | __GFP_COMP |
> > +                                           __GFP_NOWARN, order);
> > +                       ring->data[i] = page ? page_address(page) : NULL;
> > +               }
> >                 if (!ring->data[i])
> >                         return -ENOMEM;
> >         }
>
> Actually I looked closer at this driver. Is it able to receive frames
> larger than 2K? If not there isn't any point in this change.
>
> Based on commit 4fd59792097a ("net: ethernet: mediatek: support
> setting MTU") it looks like it doesn't, so odds are this patch is not
> necessary.

I spoke too soon. I had overlooked the LRO part. With that being the
case you can probably optimize this code to do away with the get_order
piece entirely, at least during runtime. My main concern is that doing
that in the fast-path will be expensive so you would be much better
off doing something like
get_order(mtk_max_frag_size(MTK_RX_FLAGS_HWLRO)) which would be
converted into a constant at compile time since everything else would
be less than 1 page in size.

Also you could then replace alloc_pages with __get_free_pages which
would take care of the page_address call for you.