On 5/8/24 6:34 AM, Yunsheng Lin wrote:
> Update documentation about design, implementation and API usages
> for page_frag.
>
> CC: Alexander Duyck <[email protected]>
> Signed-off-by: Yunsheng Lin <[email protected]>
> ---
> Documentation/mm/page_frags.rst | 156 +++++++++++++++++++++++++++++++-
> include/linux/page_frag_cache.h | 96 ++++++++++++++++++++
> mm/page_frag_cache.c | 65 ++++++++++++-
> 3 files changed, 314 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/mm/page_frags.rst b/Documentation/mm/page_frags.rst
> index 503ca6cdb804..9c25c0fd81f0 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 +42,156 @@ 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 |
> + | Cache empty v |
> + | +-----------------+ |
> + | | drain old cache | |
> + | +-----------------+ |
> + | ^ |
> + | | |
> + v v |
> + +----------------------------------+ |
> + | refill cache with order 3 page | |
> + +----------------------------------+ |
> + ^ ^ |
> + | | |
> + | | Refill failed |
> + | | | Cache is enough
> + | | |
> + | 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_alloc*_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 need to provide a aligned
needs to provide an aligned
> +fragsz if there is a alignment requirement for the size of the fragment.
an alignment
> +
> +Depending on different use cases, callers expecting to deal with va, page or
> +both va and page for them may call page_frag_alloc_va*, page_frag_alloc_pg*,
> +or page_frag_alloc* API accordingly.
> +
> +There is also a use case that need minimum memory in order for forward
needs
> +progressing, but more performant if more memory is available. Using
progress,
> +page_frag_alloc_prepare() and page_frag_alloc_commit() related API, the caller
> +requests the minimum memory it need and the prepare API will return the maximum
needs
> +size of the fragment returned, the caller needs to either call the commit API to
returned. The caller
> +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_cache_page_offset page_frag_alloc_va
> + page_frag_alloc_va_align page_frag_alloc_va_prepare_align
> + page_frag_alloc_probe page_frag_alloc_commit
> + page_frag_alloc_commit_noref
> +
> +.. kernel-doc:: mm/page_frag_cache.c
> + :identifiers: __page_frag_alloc_va_align page_frag_alloc_va_prepare
> + page_frag_alloc_pg_prepare page_frag_alloc_prepare
> + page_frag_cache_drain page_frag_free_va
> +
> +Coding examples
> +===============
> +
> +Init & Drain API
> +----------------
> +
> +.. code-block:: c
> +
> + page_frag_cache_init(pfrag);
> + ...
> + page_frag_cache_drain(pfrag);
> +
> +
> +Alloc & Free API
> +----------------
> +
> +.. code-block:: c
> +
> + void *va;
> +
> + va = page_frag_alloc_va_align(pfrag, size, gfp, align);
> + if (!va)
> + goto do_error;
> +
> + err = do_something(va, size);
> + if (err) {
> + page_frag_free_va(va);
> + goto do_error;
> + }
> +
> +Prepare & Commit API
> +--------------------
> +
> +.. code-block:: c
> +
> + unsigned int offset, size;
> + bool merge = true;
> + struct page *page;
> + void *va;
> +
> + size = 32U;
> + page = page_frag_alloc_prepare(pfrag, &offset, &size, &va);
> + if (!page)
> + goto wait_for_space;
> +
> + copy = min_t(int, copy, size);
declare copy?
> + if (!skb_can_coalesce(skb, i, page, 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_alloc_commit_noref(pfrag, offset, copy);
> + } else {
> + skb_fill_page_desc(skb, i, page, offset, copy);
> + page_frag_alloc_commit(pfrag, offset, copy);
> + }
> diff --git a/include/linux/page_frag_cache.h b/include/linux/page_frag_cache.h
> index 30893638155b..8925397262a1 100644
> --- a/include/linux/page_frag_cache.h
> +++ b/include/linux/page_frag_cache.h
> @@ -61,11 +61,28 @@ struct page_frag_cache {
> #endif
> };
>
> +/**
> + * page_frag_cache_init() - Init page_frag cache.
> + * @nc: page_frag cache from which to init
> + *
> + * Inline helper to init the page_frag cache.
> + */
> static inline void page_frag_cache_init(struct page_frag_cache *nc)
> {
> memset(nc, 0, sizeof(*nc));
> }
>
> +/**
> + * 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.
> + * It has the same calling context expection as the alloc API.
> + *
> + * Return:
> + * Return true if the current page in page_frag cache is pfmemalloc'ed,
Drop the (second) word "Return"...
> + * otherwise return false.
> + */
> static inline bool page_frag_cache_is_pfmemalloc(struct page_frag_cache *nc)
> {
> return encoded_page_pfmemalloc(nc->encoded_va);
> @@ -92,6 +109,19 @@ void *__page_frag_alloc_va_align(struct page_frag_cache *nc,
> unsigned int fragsz, gfp_t gfp_mask,
> unsigned int align_mask);
>
> +/**
> + * page_frag_alloc_va_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
needs
> + * @align: the requested aligning requirement for 'va'
or @va
> + *
> + * WARN_ON_ONCE() checking for 'align' before allocing a page fragment from
> + * page_frag cache with aligning requirement for 'va'.
or @va.
> + *
> + * Return:
> + * Return va of the page fragment, otherwise return NULL.
Drop the second "Return".
> + */
> static inline void *page_frag_alloc_va_align(struct page_frag_cache *nc,
> unsigned int fragsz,
> gfp_t gfp_mask, unsigned int align)
> @@ -100,11 +130,32 @@ static inline void *page_frag_alloc_va_align(struct page_frag_cache *nc,
> return __page_frag_alloc_va_align(nc, fragsz, gfp_mask, -align);
> }
>
> +/**
> + * 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
reasons; do not use
> + * it for new caller unless there is a strong reason.
callers
> + *
> + * Return:
> + * Return the offset of the current page fragment in the page_frag cache.
Drop second "Return".
> + */
> static inline unsigned int page_frag_cache_page_offset(const struct page_frag_cache *nc)
> {
> return __page_frag_cache_page_offset(nc->encoded_va, nc->remaining);
> }
>
> +/**
> + * page_frag_alloc_va() - 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
needs
> + *
> + * Get a page fragment from page_frag cache.
> + *
> + * Return:
> + * Return va of the page fragment, otherwise return NULL.
Drop second "Return".
> + */
> static inline void *page_frag_alloc_va(struct page_frag_cache *nc,
> unsigned int fragsz, gfp_t gfp_mask)
> {
> @@ -114,6 +165,21 @@ static inline void *page_frag_alloc_va(struct page_frag_cache *nc,
> void *page_frag_alloc_va_prepare(struct page_frag_cache *nc, unsigned int *fragsz,
> gfp_t gfp);
>
> +/**
> + * page_frag_alloc_va_prepare_align() - Prepare allocing a page fragment with
> + * aligning requirement.
> + * @nc: page_frag cache from which to prepare
> + * @fragsz: in as the requested size, out as the available size
> + * @gfp: the allocation gfp to use when cache need to be refilled
needs
> + * @align: the requested aligning requirement for 'va'
or @va
> + *
> + * WARN_ON_ONCE() checking for 'align' before preparing an aligned page fragment
> + * with minimum size of ‘fragsz’, 'fragsz' is also used to report the maximum
'fragsz'. 'fragsz' is
(don't use fancy single quote marks above)
> + * size of the page fragment the caller can use.
> + *
> + * Return:
> + * Return va of the page fragment, otherwise return NULL.
Drop second "Return".
> + */
> static inline void *page_frag_alloc_va_prepare_align(struct page_frag_cache *nc,
> unsigned int *fragsz,
> gfp_t gfp,
> @@ -148,6 +214,19 @@ static inline struct encoded_va *__page_frag_alloc_probe(struct page_frag_cache
> return encoded_va;
> }
>
> +/**
> + * page_frag_alloc_probe - Probe the avaiable page fragment.
available
> + * @nc: page_frag cache from which to probe
> + * @offset: out as the offset of the page fragment
> + * @fragsz: in as the requested size, out as the available size
> + * @va: out as the virtual address of the returned page fragment
> + *
> + * Probe the current available memory to caller without doing cache refilling.
> + * If the cache is empty, return NULL.
> + *
> + * Return:
> + * Return the page fragment, otherwise return NULL.
Drop the second "Return".
> + */
> #define page_frag_alloc_probe(nc, offset, fragsz, va) \
> ({ \
> struct encoded_va *__encoded_va; \
> @@ -162,6 +241,13 @@ static inline struct encoded_va *__page_frag_alloc_probe(struct page_frag_cache
> __page; \
> })
>
> +/**
> + * page_frag_alloc_commit - Commit allocing a page fragment.
> + * @nc: page_frag cache from which to commit
> + * @fragsz: size of the page fragment has been used
> + *
> + * Commit the alloc preparing by passing the actual used size.
> + */
> static inline void page_frag_alloc_commit(struct page_frag_cache *nc,
> unsigned int fragsz)
> {
> @@ -170,6 +256,16 @@ static inline void page_frag_alloc_commit(struct page_frag_cache *nc,
> nc->remaining -= fragsz;
> }
>
> +/**
> + * page_frag_alloc_commit_noref - Commit allocing a page fragment without taking
> + * page refcount.
> + * @nc: page_frag cache from which to commit
> + * @fragsz: size of the page fragment has been used
> + *
> + * Commit the alloc preparing by passing the actual used size, but not taking
> + * page refcount. Mostly used for fragmemt coaleasing case when the current
fragment coalescing
> + * fragmemt can share the same refcount with previous fragmemt.
fragment fragment.
> + */
> static inline void page_frag_alloc_commit_noref(struct page_frag_cache *nc,
> unsigned int fragsz)
> {
> diff --git a/mm/page_frag_cache.c b/mm/page_frag_cache.c
> index eb8bf59b26bb..85e23d5cbdcc 100644
> --- a/mm/page_frag_cache.c
> +++ b/mm/page_frag_cache.c
> @@ -89,6 +89,18 @@ static struct page *page_frag_cache_refill(struct page_frag_cache *nc,
> return __page_frag_cache_refill(nc, gfp_mask);
> }
>
> +/**
> + * page_frag_alloc_va_prepare() - Prepare allocing a page fragment.
> + * @nc: page_frag cache from which to prepare
> + * @fragsz: in as the requested size, out as the available size
> + * @gfp: the allocation gfp to use when cache need to be refilled
needs
> + *
> + * Prepare a page fragment with minimum size of ‘fragsz’, 'fragsz' is also used
'fragsz'. 'fragsz'
(don't use fancy single quote marks)
> + * to report the maximum size of the page fragment the caller can use.
> + *
> + * Return:
> + * Return va of the page fragment, otherwise return NULL.
Drop second "Return".
> + */
> void *page_frag_alloc_va_prepare(struct page_frag_cache *nc,
> unsigned int *fragsz, gfp_t gfp)
> {
> @@ -111,6 +123,19 @@ void *page_frag_alloc_va_prepare(struct page_frag_cache *nc,
> }
> EXPORT_SYMBOL(page_frag_alloc_va_prepare);
>
> +/**
> + * page_frag_alloc_pg_prepare - Prepare allocing a page fragment.
> + * @nc: page_frag cache from which to prepare
> + * @offset: out as the offset of the page fragment
> + * @fragsz: in as the requested size, out as the available size
> + * @gfp: the allocation gfp to use when cache need to be refilled
> + *
> + * Prepare a page fragment with minimum size of ‘fragsz’, 'fragsz' is also used
'fragsz'. 'fragsz'
(don't use fancy single quote marks)
> + * to report the maximum size of the page fragment the caller can use.
> + *
> + * Return:
> + * Return the page fragment, otherwise return NULL.
Drop second "Return".
> + */
> struct page *page_frag_alloc_pg_prepare(struct page_frag_cache *nc,
> unsigned int *offset,
> unsigned int *fragsz, gfp_t gfp)
> @@ -141,6 +166,21 @@ struct page *page_frag_alloc_pg_prepare(struct page_frag_cache *nc,
> }
> EXPORT_SYMBOL(page_frag_alloc_pg_prepare);
>
> +/**
> + * page_frag_alloc_prepare - Prepare allocing a page fragment.
> + * @nc: page_frag cache from which to prepare
> + * @offset: out as the offset of the page fragment
> + * @fragsz: in as the requested size, out as the available size
> + * @va: out as the virtual address of the returned page fragment
> + * @gfp: the allocation gfp to use when cache need to be refilled
> + *
> + * Prepare a page fragment with minimum size of ‘fragsz’, 'fragsz' is also used
'fragsz'. 'fragsz'
(don't use fancy single quote marks)
You could also (in several places) refer to the variables as
@fragsz. @fragsz
> + * to report the maximum size of the page fragment. Return both 'page' and 'va'
> + * of the fragment to the caller.
> + *
> + * Return:
> + * Return the page fragment, otherwise return NULL.
Drop second "Return". But the paragraph above says that both @page and @va
are returned. How is that done?
> + */
> struct page *page_frag_alloc_prepare(struct page_frag_cache *nc,
> unsigned int *offset,
> unsigned int *fragsz,
> @@ -173,6 +213,10 @@ struct page *page_frag_alloc_prepare(struct page_frag_cache *nc,
> }
> EXPORT_SYMBOL(page_frag_alloc_prepare);
>
> +/**
> + * page_frag_cache_drain - Drain the current page from page_frag cache.
> + * @nc: page_frag cache from which to drain
> + */
> void page_frag_cache_drain(struct page_frag_cache *nc)
> {
> if (!nc->encoded_va)
> @@ -193,6 +237,19 @@ void __page_frag_cache_drain(struct page *page, unsigned int count)
> }
> EXPORT_SYMBOL(__page_frag_cache_drain);
>
> +/**
> + * __page_frag_alloc_va_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'
> + *
> + * Get a page fragment from page_frag cache with aligning requirement.
> + *
> + * Return:
> + * Return va of the page fragment, otherwise return NULL.
Drop the second "Return".
> + */
> void *__page_frag_alloc_va_align(struct page_frag_cache *nc,
> unsigned int fragsz, gfp_t gfp_mask,
> unsigned int align_mask)
> @@ -263,8 +320,12 @@ void *__page_frag_alloc_va_align(struct page_frag_cache *nc,
> }
> EXPORT_SYMBOL(__page_frag_alloc_va_align);
>
> -/*
> - * Frees a page fragment allocated out of either a compound or order 0 page.
> +/**
> + * page_frag_free_va - Free a page fragment.
> + * @addr: va of page fragment to be freed
> + *
> + * Free a page fragment allocated out of either a compound or order 0 page by
> + * virtual address.
> */
> void page_frag_free_va(void *addr)
> {
thanks.
--
#Randy
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