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Message-ID: <2198afb3-4eaf-f41b-d58d-a7585f308c8c@huawei.com>
Date: Thu, 23 Nov 2023 14:18:16 +0800
From: Yunsheng Lin <linyunsheng@...wei.com>
To: Liang Chen <liangchen.linux@...il.com>, <davem@...emloft.net>,
<edumazet@...gle.com>, <kuba@...nel.org>, <pabeni@...hat.com>,
<hawk@...nel.org>, <ilias.apalodimas@...aro.org>
CC: <netdev@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH net-next v2 1/3] page_pool: Rename pp_frag_count to
pp_ref_count
On 2023/11/23 10:25, Liang Chen wrote:
> To support multiple users referencing the same fragment, pp_frag_count is
> renamed to pp_ref_count to better reflect its actual meaning based on the
> suggestion from [1].
The renaming looks good to me, some minor nit.
It is good to add a cover-letter using 'git format-patch --cover-letter'
to explain the overall background or modifications this patchset make when
there is more than one patch.
>
> [1]
> http://lore.kernel.org/netdev/f71d9448-70c8-8793-dc9a-0eb48a570300@huawei.com
>
> Signed-off-by: Liang Chen <liangchen.linux@...il.com>
> ---
> include/linux/mm_types.h | 2 +-
> include/net/page_pool/helpers.h | 31 ++++++++++++++++++-------------
> 2 files changed, 19 insertions(+), 14 deletions(-)
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 957ce38768b2..64e4572ef06d 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -125,7 +125,7 @@ struct page {
> struct page_pool *pp;
> unsigned long _pp_mapping_pad;
> unsigned long dma_addr;
> - atomic_long_t pp_frag_count;
> + atomic_long_t pp_ref_count;
It seems that we may have 4 bytes available for 64 bit arch if we change
the 'atomic_long_t' to 'refcount_t':)
> };
> struct { /* Tail pages of compound page */
> unsigned long compound_head; /* Bit zero is set */
> diff --git a/include/net/page_pool/helpers.h b/include/net/page_pool/helpers.h
> index 4ebd544ae977..a6dc9412c9ae 100644
> --- a/include/net/page_pool/helpers.h
> +++ b/include/net/page_pool/helpers.h
> @@ -29,7 +29,7 @@
> * page allocated from page pool. Page splitting enables memory saving and thus
> * avoids TLB/cache miss for data access, but there also is some cost to
> * implement page splitting, mainly some cache line dirtying/bouncing for
> - * 'struct page' and atomic operation for page->pp_frag_count.
> + * 'struct page' and atomic operation for page->pp_ref_count.
> *
> * The API keeps track of in-flight pages, in order to let API users know when
> * it is safe to free a page_pool object, the API users must call
> @@ -214,61 +214,66 @@ inline enum dma_data_direction page_pool_get_dma_dir(struct page_pool *pool)
> return pool->p.dma_dir;
> }
>
> -/* pp_frag_count represents the number of writers who can update the page
> +/* pp_ref_count represents the number of writers who can update the page
> * either by updating skb->data or via DMA mappings for the device.
> * We can't rely on the page refcnt for that as we don't know who might be
> * holding page references and we can't reliably destroy or sync DMA mappings
> * of the fragments.
> *
> - * When pp_frag_count reaches 0 we can either recycle the page if the page
> + * pp_ref_count initially corresponds to the number of fragments. However,
> + * when multiple users start to reference a single fragment, for example in
> + * skb_try_coalesce, the pp_ref_count will become greater than the number of
> + * fragments.
> + *
> + * When pp_ref_count reaches 0 we can either recycle the page if the page
> * refcnt is 1 or return it back to the memory allocator and destroy any
> * mappings we have.
> */
> static inline void page_pool_fragment_page(struct page *page, long nr)
> {
> - atomic_long_set(&page->pp_frag_count, nr);
> + atomic_long_set(&page->pp_ref_count, nr);
> }
>
> static inline long page_pool_defrag_page(struct page *page, long nr)
> {
> long ret;
>
> - /* If nr == pp_frag_count then we have cleared all remaining
> + /* If nr == pp_ref_count then we have cleared all remaining
> * references to the page:
> * 1. 'n == 1': no need to actually overwrite it.
> * 2. 'n != 1': overwrite it with one, which is the rare case
> - * for pp_frag_count draining.
> + * for pp_ref_count draining.
> *
> * The main advantage to doing this is that not only we avoid a atomic
> * update, as an atomic_read is generally a much cheaper operation than
> * an atomic update, especially when dealing with a page that may be
> - * partitioned into only 2 or 3 pieces; but also unify the pp_frag_count
> + * partitioned into only 2 or 3 pieces; but also unify the pp_ref_count
Maybe "referenced by only 2 or 3 users" is more appropriate now?
> * handling by ensuring all pages have partitioned into only 1 piece
> * initially, and only overwrite it when the page is partitioned into
> * more than one piece.
> */
> - if (atomic_long_read(&page->pp_frag_count) == nr) {
> + if (atomic_long_read(&page->pp_ref_count) == nr) {
> /* As we have ensured nr is always one for constant case using
> * the BUILD_BUG_ON(), only need to handle the non-constant case
> - * here for pp_frag_count draining, which is a rare case.
> + * here for pp_ref_count draining, which is a rare case.
> */
> BUILD_BUG_ON(__builtin_constant_p(nr) && nr != 1);
> if (!__builtin_constant_p(nr))
> - atomic_long_set(&page->pp_frag_count, 1);
> + atomic_long_set(&page->pp_ref_count, 1);
>
> return 0;
> }
>
> - ret = atomic_long_sub_return(nr, &page->pp_frag_count);
> + ret = atomic_long_sub_return(nr, &page->pp_ref_count);
> WARN_ON(ret < 0);
>
> - /* We are the last user here too, reset pp_frag_count back to 1 to
> + /* We are the last user here too, reset pp_ref_count back to 1 to
> * ensure all pages have been partitioned into 1 piece initially,
> * this should be the rare case when the last two fragment users call
> * page_pool_defrag_page() currently.
Do we need to rename the page_pool_defrag_page() and page_pool_is_last_frag()
too?
> */
> if (unlikely(!ret))
> - atomic_long_set(&page->pp_frag_count, 1);
> + atomic_long_set(&page->pp_ref_count, 1);
>
> return ret;
> }
>
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