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

From: Bagas Sanjaya
Date: Tue Oct 15 2024 - 01:20:49 EST

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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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Next message: Song Liu: "Re: [PATCH bpf-next 2/2] selftests/bpf: Extend test fs_kfuncs to cover security.bpf xattr names"
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In reply to: Yunsheng Lin: "[PATCH net-next v21 13/14] mm: page_frag: update documentation for page_frag"
Next in thread: Yunsheng Lin: "Re: [PATCH net-next v21 13/14] mm: page_frag: update documentation for page_frag"
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