linux-kernel - Re: [PATCH rfc v5 2/4] page_pool: add interface to manipulate frag count in page pool

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Message-ID: <CAKgT0UcDxSmMqCGvrWeYFiKNsxWXskF+pUhKQVCC6totduUyDQ@mail.gmail.com>
Date:   Wed, 14 Jul 2021 07:46:19 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Jesper Dangaard Brouer <jbrouer@...hat.com>
Cc:     Yunsheng Lin <linyunsheng@...wei.com>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Marcin Wojtas <mw@...ihalf.com>, linuxarm@...neuler.org,
        yisen.zhuang@...wei.com, Salil Mehta <salil.mehta@...wei.com>,
        thomas.petazzoni@...tlin.com, hawk@...nel.org,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, fenghua.yu@...el.com,
        guro@...com, Peter Xu <peterx@...hat.com>,
        Feng Tang <feng.tang@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Matteo Croce <mcroce@...rosoft.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Alexander Lobakin <alobakin@...me>,
        Willem de Bruijn <willemb@...gle.com>, wenxu@...oud.cn,
        Cong Wang <cong.wang@...edance.com>,
        Kevin Hao <haokexin@...il.com>, nogikh@...gle.com,
        Marco Elver <elver@...gle.com>, Yonghong Song <yhs@...com>,
        kpsingh@...nel.org, andrii@...nel.org,
        Martin KaFai Lau <kafai@...com>, songliubraving@...com,
        Netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH rfc v5 2/4] page_pool: add interface to manipulate frag
 count in page pool

On Wed, Jul 14, 2021 at 3:18 AM Jesper Dangaard Brouer
<jbrouer@...hat.com> wrote:
>
>
>
> On 14/07/2021 11.34, Yunsheng Lin wrote:
> > As suggested by Alexander, "A DMA mapping should be page
> > aligned anyway so the lower 12 bits would be reserved 0",
> > so it might make more sense to repurpose the lower 12 bits
> > of the dma address to store the frag count for frag page
> > support in page pool for 32 bit systems with 64 bit dma,
> > which should be rare those days.
>
> Do we have any real driver users with 32-bit arch and 64-bit DMA, that
> want to use this new frag-count system you are adding to page_pool?
>
> This "lower 12-bit use" complicates the code we need to maintain
> forever. My guess is that it is never used, but we need to update and
> maintain it, and it will never be tested.
>
> Why don't you simply reject using page_pool flag PP_FLAG_PAGE_FRAG
> during setup of the page_pool for this case?
>
>   if ((pool->p.flags & PP_FLAG_PAGE_FRAG) &&
>       (sizeof(dma_addr_t) > sizeof(unsigned long)))
>     goto reject-setup;
>
>

That sounds good to me if we want to go that route. It would simplify
this quite a bit since essentially we could just drop these if blocks.

Thanks.

- Alex