netdev - Re: [PATCH net-next v7 1/4] page_pool: transition to reference count management after page draining

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Message-ID: <20231208173816.2f32ad0f@kernel.org>
Date: Fri, 8 Dec 2023 17:38:16 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Liang Chen <liangchen.linux@...il.com>
Cc: davem@...emloft.net, edumazet@...gle.com, pabeni@...hat.com,
 hawk@...nel.org, ilias.apalodimas@...aro.org, linyunsheng@...wei.com,
 netdev@...r.kernel.org, linux-mm@...ck.org, jasowang@...hat.com
Subject: Re: [PATCH net-next v7 1/4] page_pool: transition to reference
 count management after page draining

On Wed,  6 Dec 2023 18:54:16 +0800 Liang Chen wrote:
> -/* 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.
>   */

Sorry to nit pick but I think this whole doc has to be rewritten
completely. It does state the most important thing which is that
the caller must have just allocated the page.

How about:

/**
 * page_pool_fragment_page() - split a fresh page into fragments
 * @.. fill these in
 *
 * pp_ref_count represents the number of outstanding references
 * to the page, which will be freed using page_pool APIs (rather
 * than page allocator APIs like put_page()). Such references are
 * usually held by page_pool-aware objects like skbs marked for
 * page pool recycling.
 *
 * This helper allows the caller to take (set) multiple references
 * to a freshly allocated page. The page must be freshly allocated
 * (have a pp_ref_count of 1). This is commonly done by drivers
 * and "fragment allocators" to save atomic operations - either
 * when they know upfront how many references they will need; or
 * to take MAX references and return the unused ones with a single
 * atomic dec(), instead of performing multiple atomic inc()
 * operations.
 */

I think that's more informative at this stage of evolution of
the  page pool API, when most users aren't experts on internals.
But feel free to disagree..

>  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);
>  }

The code itself and rest of the patches LGTM, although it would be
great to get ACKs from pp maintainers..