Re: [PATCH net-next v22 13/14] mm: page_frag: update documentation for page_frag

[Bagas Sanjaya <bagasdotme@gmail.com>]

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On Fri, Oct 18, 2024 at 06:53:50PM +0800, Yunsheng Lin wrote:
> diff --git a/Documentation/mm/page_frags.rst b/Documentation/mm/page_frags.rst
> index 503ca6cdb804..7fd9398aca4e 100644
> --- a/Documentation/mm/page_frags.rst
> +++ b/Documentation/mm/page_frags.rst
> @@ -1,3 +1,5 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
>  ==============
>  Page fragments
>  ==============
> @@ -40,4 +42,176 @@ page via a single call.  The advantage to doing this is that it allows for
>  cleaning up the multiple references that were added to a page in order to
>  avoid calling get_page per allocation.
>  
> -Alexander Duyck, Nov 29, 2016.
> +
> +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.
> +
> +.. kernel-doc:: include/linux/page_frag_cache.h
> +   :identifiers: page_frag_cache_init page_frag_cache_is_pfmemalloc
> +		  __page_frag_alloc_align page_frag_alloc_align page_frag_alloc
> +		 __page_frag_refill_align page_frag_refill_align
> +		 page_frag_refill __page_frag_refill_prepare_align
> +		 page_frag_refill_prepare_align page_frag_refill_prepare
> +		 __page_frag_alloc_refill_prepare_align
> +		 page_frag_alloc_refill_prepare_align
> +		 page_frag_alloc_refill_prepare page_frag_alloc_refill_probe
> +		 page_frag_refill_probe page_frag_commit
> +		 page_frag_commit_noref page_frag_alloc_abort
> +
> +.. kernel-doc:: mm/page_frag_cache.c
> +   :identifiers: page_frag_cache_drain page_frag_free
> +		 __page_frag_alloc_refill_probe_align
> +
> +Coding examples
> +===============
> +
> +Initialization and draining API
> +-------------------------------
> +
> +.. code-block:: c
> +
> +   page_frag_cache_init(nc);
> +   ...
> +   page_frag_cache_drain(nc);
> +
> +
> +Allocation & freeing API
> +------------------------
> +
> +.. code-block:: c
> +
> +    void *va;
> +
> +    va = page_frag_alloc_align(nc, size, gfp, align);
> +    if (!va)
> +        goto do_error;
> +
> +    err = do_something(va, size);
> +    if (err) {
> +        page_frag_abort(nc, size);
> +        goto do_error;
> +    }
> +
> +    ...
> +
> +    page_frag_free(va);
> +
> +
> +Preparation & committing API
> +----------------------------
> +
> +.. code-block:: c
> +
> +    struct page_frag page_frag, *pfrag;
> +    bool merge = true;
> +    void *va;
> +
> +    pfrag = &page_frag;
> +    va = page_frag_alloc_refill_prepare(nc, 32U, pfrag, GFP_KERNEL);
> +    if (!va)
> +        goto wait_for_space;
> +
> +    copy = min_t(unsigned int, copy, pfrag->size);
> +    if (!skb_can_coalesce(skb, i, pfrag->page, pfrag->offset)) {
> +        if (i >= max_skb_frags)
> +            goto new_segment;
> +
> +        merge = false;
> +    }
> +
> +    copy = mem_schedule(copy);
> +    if (!copy)
> +        goto wait_for_space;
> +
> +    err = copy_from_iter_full_nocache(va, copy, iter);
> +    if (err)
> +        goto do_error;
> +
> +    if (merge) {
> +        skb_frag_size_add(&skb_shinfo(skb)->frags[i - 1], copy);
> +        page_frag_commit_noref(nc, pfrag, copy);
> +    } else {
> +        skb_fill_page_desc(skb, i, pfrag->page, pfrag->offset, copy);
> +        page_frag_commit(nc, pfrag, copy);
> +    }

Looks good.

> +/**
> + * 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 allocated from the
"Check if ..."
> + * pfmemalloc reserves. It has the same calling context expectation as the
> + * allocation API.
> + *
> + * Return:
> + * true if the current page in page_frag cache is allocated from the pfmemalloc
> + * reserves, otherwise return false.
> + */
> <snipped>...
> +/**
> + * page_frag_alloc() - Allocate 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 a page fragment ..."
> + *
> + * Return:
> + * virtual address of the page fragment, otherwise return NULL.
> + */
>  static inline void *page_frag_alloc(struct page_frag_cache *nc,
> <snipped>...
> +/**
> + * __page_frag_refill_prepare_align() - Prepare refilling a page_frag with
> + * aligning requirement.
> + * @nc: page_frag cache from which to 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 refill a page_frag from page_frag cache with aligning requirement.
"Prepare refilling ..."
> + *
> + * Return:
> + * True if prepare refilling succeeds, otherwise return false.
> + */
> <snipped>...
> +/**
> + * __page_frag_alloc_refill_probe_align() - Probe 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.
> + * @align_mask: the requested aligning requirement for the fragment.
> + *
> + * Probe allocing a fragment and refilling a page_frag from page_frag cache with
"Probe allocating..."
> + * aligning requirement.
> + *
> + * Return:
> + * virtual address of the page fragment, otherwise return NULL.
> + */

Thanks.

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

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