Re: [PATCH net-next v21 13/14] mm: page_frag: update documentation for page_frag

[Bagas Sanjaya <bagasdotme@gmail.com>]

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On Sat, Oct 12, 2024 at 07:23:19PM +0800, Yunsheng Lin wrote:
> +Architecture overview
> +=====================
> +
> +.. code-block:: none
> +
> +                      +----------------------+
> +                      | page_frag API caller |
> +                      +----------------------+
> +                                  |
> +                                  |
> +                                  v
> +    +------------------------------------------------------------------+
> +    |                   request page fragment                          |
> +    +------------------------------------------------------------------+
> +             |                                 |                     |
> +             |                                 |                     |
> +             |                          Cache not enough             |
> +             |                                 |                     |
> +             |                         +-----------------+           |
> +             |                         | reuse old cache |--Usable-->|
> +             |                         +-----------------+           |
> +             |                                 |                     |
> +             |                             Not usable                |
> +             |                                 |                     |
> +             |                                 v                     |
> +        Cache empty                   +-----------------+            |
> +             |                        | drain old cache |            |
> +             |                        +-----------------+            |
> +             |                                 |                     |
> +             v_________________________________v                     |
> +                              |                                      |
> +                              |                                      |
> +             _________________v_______________                       |
> +            |                                 |              Cache is enough
> +            |                                 |                      |
> + PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE         |                      |
> +            |                                 |                      |
> +            |               PAGE_SIZE >= PAGE_FRAG_CACHE_MAX_SIZE    |
> +            v                                 |                      |
> +    +----------------------------------+      |                      |
> +    | refill cache with order > 0 page |      |                      |
> +    +----------------------------------+      |                      |
> +      |                    |                  |                      |
> +      |                    |                  |                      |
> +      |              Refill failed            |                      |
> +      |                    |                  |                      |
> +      |                    v                  v                      |
> +      |      +------------------------------------+                  |
> +      |      |   refill cache with order 0 page   |                  |
> +      |      +----------------------------------=-+                  |
> +      |                       |                                      |
> + Refill succeed               |                                      |
> +      |                 Refill succeed                               |
> +      |                       |                                      |
> +      v                       v                                      v
> +    +------------------------------------------------------------------+
> +    |             allocate fragment from cache                         |
> +    +------------------------------------------------------------------+
> +
> +API interface
> +=============
> +As the design and implementation of page_frag API implies, the allocation side
> +does not allow concurrent calling. Instead it is assumed that the caller must
> +ensure there is not concurrent alloc calling to the same page_frag_cache
> +instance by using its own lock or rely on some lockless guarantee like NAPI
> +softirq.
> +
> +Depending on different aligning requirement, the page_frag API caller may call
> +page_frag_*_align*() to ensure the returned virtual address or offset of the
> +page is aligned according to the 'align/alignment' parameter. Note the size of
> +the allocated fragment is not aligned, the caller needs to provide an aligned
> +fragsz if there is an alignment requirement for the size of the fragment.
> +
> +Depending on different use cases, callers expecting to deal with va, page or
> +both va and page for them may call page_frag_alloc, page_frag_refill, or
> +page_frag_alloc_refill API accordingly.
> +
> +There is also a use case that needs minimum memory in order for forward progress,
> +but more performant if more memory is available. Using page_frag_*_prepare() and
> +page_frag_commit*() related API, the caller requests the minimum memory it needs
> +and the prepare API will return the maximum size of the fragment returned. The
> +caller needs to either call the commit API to report how much memory it actually
> +uses, or not do so if deciding to not use any memory.

Looks OK.

> +Coding examples
> +===============
> +
> +Init & Drain API
> +----------------

Initialization and draining API?

> +Alloc & Free API
> +----------------

Shouldn't it be called allocation API?

> +Prepare & Commit API
> +--------------------

This one looks OK.

> +/**
> + * page_frag_cache_init() - Init page_frag cache.
> + * @nc: page_frag cache from which to init
> + *
> + * Inline helper to init the page_frag cache.
> + */
s/Init/Initialize/
>  static inline void page_frag_cache_init(struct page_frag_cache *nc)
>  {
>  	nc->encoded_page = 0;
>  }
>  
> +/**
> + * page_frag_cache_is_pfmemalloc() - Check for pfmemalloc.
> + * @nc: page_frag cache from which to check
> + *
> + * Used to check if the current page in page_frag cache is pfmemalloc'ed.
                                                           is allocated by pfmemalloc()?
> + * It has the same calling context expectation as the alloc API.
> + *
> + * Return:
> + * true if the current page in page_frag cache is pfmemalloc'ed, otherwise
> + * return false.
> + */
>  static inline bool page_frag_cache_is_pfmemalloc(struct page_frag_cache *nc)
>  {
>  	return encoded_page_decode_pfmemalloc(nc->encoded_page);
>  }
>  
> +/**
> + * page_frag_cache_page_offset() - Return the current page fragment's offset.
> + * @nc: page_frag cache from which to check
> + *
> + * The API is only used in net/sched/em_meta.c for historical reason, do not use
> + * it for new caller unless there is a strong reason.

Then what does page_frag_cache_page_offset() do then?

> + *
> + * Return:
> + * the offset of the current page fragment in the page_frag cache.
> + */
>  static inline unsigned int page_frag_cache_page_offset(const struct page_frag_cache *nc)
>  {
>  	return nc->offset;
> @@ -66,6 +93,19 @@ static inline unsigned int __page_frag_cache_commit(struct page_frag_cache *nc,
>  	return __page_frag_cache_commit_noref(nc, pfrag, used_sz);
>  }
>  
> +/**
> + * __page_frag_alloc_align() - Alloc a page fragment with aligning
> + * requirement.
> + * @nc: page_frag cache from which to allocate
> + * @fragsz: the requested fragment size
> + * @gfp_mask: the allocation gfp to use when cache need to be refilled
> + * @align_mask: the requested aligning requirement for the 'va'
> + *
> + * Alloc a page fragment from page_frag cache with aligning requirement.
      Allocate
> + *
> + * Return:
> + * Virtual address of the page fragment, otherwise return NULL.
> + */
>  static inline void *__page_frag_alloc_align(struct page_frag_cache *nc,
>  					    unsigned int fragsz, gfp_t gfp_mask,
>  					    unsigned int align_mask)
> @@ -83,6 +123,19 @@ static inline void *__page_frag_alloc_align(struct page_frag_cache *nc,
>  	return va;
>  }
>  
> +/**
> + * page_frag_alloc_align() - Alloc a page fragment with aligning requirement.
> + * @nc: page_frag cache from which to allocate
> + * @fragsz: the requested fragment size
> + * @gfp_mask: the allocation gfp to use when cache needs to be refilled
> + * @align: the requested aligning requirement for the fragment
> + *
> + * WARN_ON_ONCE() checking for @align before allocing a page fragment from
                                                allocating
> + * page_frag cache with aligning requirement.
> + *
> + * Return:
> + * virtual address of the page fragment, otherwise return NULL.
> + */
>  static inline void *page_frag_alloc_align(struct page_frag_cache *nc,
>  					  unsigned int fragsz, gfp_t gfp_mask,
>  					  unsigned int align)
> @@ -91,12 +144,36 @@ static inline void *page_frag_alloc_align(struct page_frag_cache *nc,
>  	return __page_frag_alloc_align(nc, fragsz, gfp_mask, -align);
>  }
>  
> +/**
> + * page_frag_alloc() - Alloc a page fragment.
> + * @nc: page_frag cache from which to allocate
> + * @fragsz: the requested fragment size
> + * @gfp_mask: the allocation gfp to use when cache need to be refilled
> + *
> + * Alloc a page fragment from page_frag cache.
      Allocate
> + *
> + * Return:
> + * virtual address of the page fragment, otherwise return NULL.
> + */
>  static inline void *page_frag_alloc(struct page_frag_cache *nc,
>  				    unsigned int fragsz, gfp_t gfp_mask)
>  {
>  	return __page_frag_alloc_align(nc, fragsz, gfp_mask, ~0u);
>  }
>  
> +/**
> + * __page_frag_alloc_refill_prepare_align() - Prepare allocing a fragment and
> + * refilling a page_frag with aligning requirement.
> + * @nc: page_frag cache from which to allocate and refill
> + * @fragsz: the requested fragment size
> + * @pfrag: the page_frag to be refilled.
> + * @gfp_mask: the allocation gfp to use when cache need to be refilled
> + * @align_mask: the requested aligning requirement for the fragment.
> + *
> + * Prepare allocing a fragment and refilling a page_frag from page_frag cache.
      Prepare allocating?
> + *
> + * Return:
> + * virtual address of the page fragment, otherwise return NULL.
> + */
>  static inline void *__page_frag_alloc_refill_prepare_align(struct page_frag_cache *nc,
>  							   unsigned int fragsz,
>  							   struct page_frag *pfrag,
> @@ -166,6 +324,21 @@ static inline void *__page_frag_alloc_refill_prepare_align(struct page_frag_cach
>  	return __page_frag_cache_prepare(nc, fragsz, pfrag, gfp_mask, align_mask);
>  }

Thanks.

-- 
An old man doll... just what I always wanted! - Clara

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