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Date: Tue, 10 Aug 2021 16:58:05 +0200
From: Jesper Dangaard Brouer <jbrouer@...hat.com>
To: Yunsheng Lin <linyunsheng@...wei.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,
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ilias.apalodimas@...aro.org, ast@...nel.org, daniel@...earbox.net,
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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 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
> +
> 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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