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: <73ef1add-01df-7783-7982-da9e59c26330@huawei.com>
Date:   Thu, 15 Jul 2021 11:49:56 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Alexander Duyck <alexander.duyck@...il.com>,
        Jesper Dangaard Brouer <jbrouer@...hat.com>
CC:     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 2021/7/14 22:46, Alexander Duyck wrote:
> 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.

Ok, let's wait for a few day to see if there is anyone with 32-bit arch
and 64-bit DMA system care enough to use the frag-count support in the page
pool.

> 
> Thanks.
> 
> - Alex
> .
>