linux-kernel - Re: [PATCH rfc v6 2/4] page_pool: add interface to manipulate frag count in page pool

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Date:   Tue, 20 Jul 2021 08:43:42 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        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 v6 2/4] page_pool: add interface to manipulate frag
 count in page pool

On Mon, Jul 19, 2021 at 8:36 PM Yunsheng Lin <linyunsheng@...wei.com> wrote:
>
> For 32 bit systems with 64 bit dma, dma_addr[1] is used to
> store the upper 32 bit dma addr, those system should be rare
> those days.
>
> For normal system, the dma_addr[1] in 'struct page' is not
> used, so we can reuse dma_addr[1] for storing frag count,
> which means how many frags this page might be splited to.
>
> In order to simplify the page frag support in the page pool,
> the PAGE_POOL_DMA_USE_PP_FRAG_COUNT macro is added to indicate
> the 32 bit systems with 64 bit dma, and the page frag support
> in page pool is disabled for such system.
>
> The newly added page_pool_set_frag_count() is called to reserve
> the maximum frag count before any page frag is passed to the
> user. The page_pool_atomic_sub_frag_count_return() is called
> when user is done with the page frag.
>
> Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
> ---
>  include/linux/mm_types.h | 18 +++++++++++++-----
>  include/net/page_pool.h  | 41 ++++++++++++++++++++++++++++++++++-------
>  net/core/page_pool.c     |  4 ++++
>  3 files changed, 51 insertions(+), 12 deletions(-)
>

<snip>

> +static inline long page_pool_atomic_sub_frag_count_return(struct page *page,
> +                                                         long nr)
> +{
> +       long frag_count = atomic_long_read(&page->pp_frag_count);
> +       long ret;
> +
> +       if (frag_count == nr)
> +               return 0;
> +
> +       ret = atomic_long_sub_return(nr, &page->pp_frag_count);
> +       WARN_ON(ret < 0);
> +       return ret;
>  }
>

So this should just be an atomic_long_sub_return call. You should get
rid of the atomic_long_read portion of this as it can cover up
reference count errors.