linux-kernel - Re:Re: [PATCH v2] net: ethernet: mtk_eth_soc: fix misuse of mem alloc interface netdev[napi]_alloc

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Date:   Sun, 5 Jun 2022 10:22:26 +0800 (CST)
From:   "Chen Lin" <chen45464546@....com>
To:     "Alexander Duyck" <alexander.duyck@...il.com>
Cc:     "Felix Fietkau" <nbd@....name>, "Jakub Kicinski" <kuba@...nel.org>,
        "John Crispin" <john@...ozen.org>,
        "Sean Wang" <sean.wang@...iatek.com>, Mark-MC.Lee@...iatek.com,
        "David Miller" <davem@...emloft.net>,
        "Eric Dumazet" <edumazet@...gle.com>,
        "Paolo Abeni" <pabeni@...hat.com>, matthias.bgg@...il.com,
        Netdev <netdev@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re:Re: [PATCH v2] net: ethernet: mtk_eth_soc: fix misuse of mem
 alloc interface netdev[napi]_alloc_frag

At 2022-06-03 23:33:25, "Alexander Duyck" <alexander.duyck@...il.com> wrote:
>On Fri, Jun 3, 2022 at 8:25 AM Alexander Duyck
><alexander.duyck@...il.com> wrote:
>>
>> On Fri, Jun 3, 2022 at 2:03 AM Chen Lin <chen45464546@....com> 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@....com>
>> > ---
>> >  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.

Thanks for the tips. I'll try again.
It can also be seen from here it is easy to make mistakes in parameter fragsz.