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

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2024/10/20 18:02, Bagas Sanjaya wrote:

Thanks, will try my best to not miss any 'alloc' typo for doc patch
next version:(

> 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.
>