linux-kernel - Re: [PATCH rfc v3 2/4] page_pool: add interface for getting and setting pagecnt

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Message-ID: <2d9a3d29-8e6b-8462-c410-6b7fd4518c9d@redhat.com>
Date:   Mon, 12 Jul 2021 19:20:46 +0200
From:   Jesper Dangaard Brouer <jbrouer@...hat.com>
To:     Alexander Duyck <alexander.duyck@...il.com>,
        Yunsheng Lin <linyunsheng@...wei.com>
Cc:     brouer@...hat.com, 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 v3 2/4] page_pool: add interface for getting and
 setting pagecnt_bias



On 12/07/2021 18.02, Alexander Duyck wrote:
> On Mon, Jul 12, 2021 at 5:17 AM Yunsheng Lin <linyunsheng@...wei.com> 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 pagecnt_bias for frag page
>> support in page pool.
>>
>> As newly added page_pool_get_pagecnt_bias() may be called
>> outside of the softirq context, so annotate the access to
>> page->dma_addr[0] with READ_ONCE() and WRITE_ONCE().
>>
>> And page_pool_get_pagecnt_bias_ptr() is added to implement
>> the pagecnt_bias atomic updating when a page is passsed to
>> the user.
>>
>> Other three interfaces using page->dma_addr[0] is only called
>> in the softirq context during normal rx processing, hopefully
>> the barrier in the rx processing will ensure the correct order
>> between getting and setting pagecnt_bias.
>>
>> Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
>> ---
>>   include/net/page_pool.h | 29 +++++++++++++++++++++++++++--
>>   net/core/page_pool.c    |  8 +++++++-
>>   2 files changed, 34 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/net/page_pool.h b/include/net/page_pool.h
>> index 8d7744d..84cd972 100644
>> --- a/include/net/page_pool.h
>> +++ b/include/net/page_pool.h
>> @@ -200,17 +200,42 @@ static inline void page_pool_recycle_direct(struct page_pool *pool,
>>
>>   static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
>>   {
>> -       dma_addr_t ret = page->dma_addr[0];
>> +       dma_addr_t ret = READ_ONCE(page->dma_addr[0]) & PAGE_MASK;
>>          if (sizeof(dma_addr_t) > sizeof(unsigned long))
>>                  ret |= (dma_addr_t)page->dma_addr[1] << 16 << 16;
>>          return ret;
>>   }
>>
>> -static inline void page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
>> +static inline bool page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
>>   {
>> +       if (WARN_ON(addr & ~PAGE_MASK))
>> +               return false;
>> +
>>          page->dma_addr[0] = addr;
>>          if (sizeof(dma_addr_t) > sizeof(unsigned long))
>>                  page->dma_addr[1] = upper_32_bits(addr);
>> +
>> +       return true;
>> +}
>> +
> 
> Rather than making this a part of the check here it might make more
> sense to pull this out and perform the WARN_ON after the check for
> dma_mapping_error.

I need to point out that I don't like WARN_ON and BUG_ON code in 
fast-path code, because compiler adds 'ud2' assembler instructions that 
influences the instruction-cache fetching in the CPU.  Yes, I have seen 
a measuresable impact from this before.


> Also it occurs to me that we only really have to do this in the case
> where dma_addr_t is larger than the size of a long. Otherwise we could
> just have the code split things so that dma_addr[0] is the dma_addr
> and dma_addr[1] is our pagecnt_bias value in which case we could
> probably just skip the check.

The dance to get 64-bit DMA addr on 32-bit systems is rather ugly and 
confusing, sadly.  We could take advantage of this, I just hope this 
will not make it uglier.


>> +static inline int page_pool_get_pagecnt_bias(struct page *page)
>> +{
>> +       return READ_ONCE(page->dma_addr[0]) & ~PAGE_MASK;
>> +}
>> +
>> +static inline unsigned long *page_pool_pagecnt_bias_ptr(struct page *page)
>> +{
>> +       return page->dma_addr;
>> +}
>> +
>> +static inline void page_pool_set_pagecnt_bias(struct page *page, int bias)
>> +{
>> +       unsigned long dma_addr_0 = READ_ONCE(page->dma_addr[0]);
>> +
>> +       dma_addr_0 &= PAGE_MASK;
>> +       dma_addr_0 |= bias;
>> +
>> +       WRITE_ONCE(page->dma_addr[0], dma_addr_0);
>>   }
>>
>>   static inline bool is_page_pool_compiled_in(void)
>> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
>> index 78838c6..1abefc6 100644
>> --- a/net/core/page_pool.c
>> +++ b/net/core/page_pool.c
>> @@ -198,7 +198,13 @@ static bool page_pool_dma_map(struct page_pool *pool, struct page *page)
>>          if (dma_mapping_error(pool->p.dev, dma))
>>                  return false;
>>
> 
> So instead of adding to the function below you could just add your
> WARN_ON check here with the unmapping call.
> 
>> -       page_pool_set_dma_addr(page, dma);
>> +       if (unlikely(!page_pool_set_dma_addr(page, dma))) {
>> +               dma_unmap_page_attrs(pool->p.dev, dma,
>> +                                    PAGE_SIZE << pool->p.order,
>> +                                    pool->p.dma_dir,
>> +                                    DMA_ATTR_SKIP_CPU_SYNC);
>> +               return false;
>> +       }
>>
>>          if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
>>                  page_pool_dma_sync_for_device(pool, page, pool->p.max_len);
>> --
>> 2.7.4
>>
>