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Date: Wed, 11 Aug 2021 08:48:55 +0800
From: Yunsheng Lin <linyunsheng@...wei.com>
To: Jesper Dangaard Brouer <jbrouer@...hat.com>, <davem@...emloft.net>,
<kuba@...nel.org>
CC: <brouer@...hat.com>, <alexander.duyck@...il.com>,
<linux@...linux.org.uk>, <mw@...ihalf.com>,
<linuxarm@...neuler.org>, <yisen.zhuang@...wei.com>,
<salil.mehta@...wei.com>, <thomas.petazzoni@...tlin.com>,
<hawk@...nel.org>, <ilias.apalodimas@...aro.org>, <ast@...nel.org>,
<daniel@...earbox.net>, <john.fastabend@...il.com>,
<akpm@...ux-foundation.org>, <peterz@...radead.org>,
<will@...nel.org>, <willy@...radead.org>, <vbabka@...e.cz>,
<fenghua.yu@...el.com>, <guro@...com>, <peterx@...hat.com>,
<feng.tang@...el.com>, <jgg@...pe.ca>, <mcroce@...rosoft.com>,
<hughd@...gle.com>, <jonathan.lemon@...il.com>, <alobakin@...me>,
<willemb@...gle.com>, <wenxu@...oud.cn>, <cong.wang@...edance.com>,
<haokexin@...il.com>, <nogikh@...gle.com>, <elver@...gle.com>,
<yhs@...com>, <kpsingh@...nel.org>, <andrii@...nel.org>,
<kafai@...com>, <songliubraving@...com>, <netdev@...r.kernel.org>,
<linux-kernel@...r.kernel.org>, <bpf@...r.kernel.org>,
<chenhao288@...ilicon.com>, Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH net-next v2 2/4] page_pool: add interface to manipulate
frag count in page pool
On 2021/8/10 22:58, Jesper Dangaard Brouer wrote:
>
>
> On 06/08/2021 04.46, Yunsheng Lin 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 | 46 +++++++++++++++++++++++++++++++++++++++-------
>> net/core/page_pool.c | 4 ++++
>> 3 files changed, 56 insertions(+), 12 deletions(-)
>>
>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>> index 52bbd2b..7f8ee09 100644
>> --- a/include/linux/mm_types.h
>> +++ b/include/linux/mm_types.h
>> @@ -103,11 +103,19 @@ struct page {
>> unsigned long pp_magic;
>> struct page_pool *pp;
>> unsigned long _pp_mapping_pad;
>> - /**
>> - * @dma_addr: might require a 64-bit value on
>> - * 32-bit architectures.
>> - */
>> - unsigned long dma_addr[2];
>> + unsigned long dma_addr;
>> + union {
>> + /**
>> + * dma_addr_upper: might require a 64-bit
>> + * value on 32-bit architectures.
>> + */
>> + unsigned long dma_addr_upper;
>> + /**
>> + * For frag page support, not supported in
>> + * 32-bit architectures with 64-bit DMA.
>> + */
>> + atomic_long_t pp_frag_count;
>> + };
>> };
>> struct { /* slab, slob and slub */
>> union {
>> diff --git a/include/net/page_pool.h b/include/net/page_pool.h
>> index 8d7744d..42e6997 100644
>> --- a/include/net/page_pool.h
>> +++ b/include/net/page_pool.h
>> @@ -45,7 +45,10 @@
>> * Please note DMA-sync-for-CPU is still
>> * device driver responsibility
>> */
>> -#define PP_FLAG_ALL (PP_FLAG_DMA_MAP | PP_FLAG_DMA_SYNC_DEV)
>> +#define PP_FLAG_PAGE_FRAG BIT(2) /* for page frag feature */
>> +#define PP_FLAG_ALL (PP_FLAG_DMA_MAP |\
>> + PP_FLAG_DMA_SYNC_DEV |\
>> + PP_FLAG_PAGE_FRAG)
>> /*
>> * Fast allocation side cache array/stack
>> @@ -198,19 +201,48 @@ static inline void page_pool_recycle_direct(struct page_pool *pool,
>> page_pool_put_full_page(pool, page, true);
>> }
>> +#define PAGE_POOL_DMA_USE_PP_FRAG_COUNT \
>> + (sizeof(dma_addr_t) > sizeof(unsigned long))
>> +
>> static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
>> {
>> - dma_addr_t ret = page->dma_addr[0];
>> - if (sizeof(dma_addr_t) > sizeof(unsigned long))
>> - ret |= (dma_addr_t)page->dma_addr[1] << 16 << 16;
>> + dma_addr_t ret = page->dma_addr;
>> +
>> + if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
>> + ret |= (dma_addr_t)page->dma_addr_upper << 16 << 16;
>
> I find the macro name confusing.
>
> I think it would be easier to read the code, if it was called:
> PAGE_POOL_DMA_CANNOT_USE_PP_FRAG_COUNT
Actually, there is a *DMA* in tha above macro, which means DMA
addr uses the PP_FRAG_COUNT field.
Perhaps PAGE_POOL_DMA_ADDR_UPPER_USE_PP_FRAG_COUNT is more obvious
here?
>
>> +
>> return ret;
>> }
>> static inline void page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
>> {
>> - page->dma_addr[0] = addr;
>> - if (sizeof(dma_addr_t) > sizeof(unsigned long))
>> - page->dma_addr[1] = upper_32_bits(addr);
>> + page->dma_addr = addr;
>> + if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
>> + page->dma_addr_upper = upper_32_bits(addr);
>> +}
>> +
>> +static inline void page_pool_set_frag_count(struct page *page, long nr)
>> +{
>> + atomic_long_set(&page->pp_frag_count, nr);
>> +}
>> +
>> +static inline long page_pool_atomic_sub_frag_count_return(struct page *page,
>> + long nr)
>> +{
>> + long ret;
>> +
>> + /* As suggested by Alexander, atomic_long_read() may cover up the
>> + * reference count errors, so avoid calling atomic_long_read() in
>> + * the cases of freeing or draining the page_frags, where we would
>> + * not expect it to match or that are slowpath anyway.
>> + */
>> + if (__builtin_constant_p(nr) &&
>> + atomic_long_read(&page->pp_frag_count) == nr)
>> + return 0;
>> +
>> + ret = atomic_long_sub_return(nr, &page->pp_frag_count);
>> + WARN_ON(ret < 0);
>> + return ret;
>> }
>> 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..68fab94 100644
>> --- a/net/core/page_pool.c
>> +++ b/net/core/page_pool.c
>> @@ -67,6 +67,10 @@ static int page_pool_init(struct page_pool *pool,
>> */
>> }
>> + if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT &&
>> + pool->p.flags & PP_FLAG_PAGE_FRAG)
>> + return -EINVAL;
>
> I read this as: if the page_pool use pp_frag_count and have flag set, then it is invalid/no-allowed, which seems wrong.
>
> I find this code more intuitive to read:
>
> + if (PAGE_POOL_DMA_CANNOT_USE_PP_FRAG_COUNT &&
> + pool->p.flags & PP_FLAG_PAGE_FRAG)
> + return -EINVAL;
>
> --Jesper
>
> .
>
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