On Wed, 8 May 2024, Yunsheng Lin wrote:
Update documentation about design, implementation and API usages
for page_frag.
CC: Alexander Duyck <alexander.duyck@xxxxxxxxx>
Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
---
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
+fragsz if there is a 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_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
+progressing, but more performant if more memory is available. Using
+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
+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_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);
+ 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,
+ * 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
+ * @align: the requested aligning requirement for 'va'
+ *
+ * WARN_ON_ONCE() checking for 'align' before allocing a page fragment from
+ * page_frag cache with aligning requirement for 'va'.
+ *
+ * Return:
+ * Return va of the page fragment, otherwise return NULL.
+ */
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
+ * it for new caller unless there is a strong reason.
+ *
+ * Return:
+ * 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 __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
+ *
+ * Get a page fragment from page_frag cache.
+ *
+ * Return:
+ * Return va of the page fragment, otherwise return NULL.
+ */
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
+ * @align: the requested aligning requirement for '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
+ * size of the page fragment the caller can use.
+ *
+ * Return:
+ * Return va of the page fragment, otherwise return NULL.
+ */
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.
+ * @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
Hi Yunsheng -
fragsz is never used as an input in this function. I think it would be
good to make the code consistent with this documentation by checking that
*fragsz <= (nc)->remaining
+ * @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.
Instead of this line, is it more accurate to say "if no space is available
in the page_frag cache, return NULL" ?
I also suggest adding some documentation here like:
"If the requested space is available, up to fragsz bytes may be added to
the fragment using page_frag_alloc_commit()".
+ *
+ * Return:
+ * Return the page fragment, otherwise return NULL.
+ */
#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.
Rephrasing suggestion:
"Commit the actual used size for the allocation that was either prepared
or probed"
Thanks,
Mat
+ */
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
+ * fragmemt can share the same refcount with previous fragmemt.
+ */
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
+ *
+ * Prepare a page fragment with minimum size of ???fragsz???, 'fragsz' is also used
+ * to report the maximum size of the page fragment the caller can use.
+ *
+ * Return:
+ * Return va of the page fragment, otherwise return NULL.
+ */
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
+ * to report the maximum size of the page fragment the caller can use.
+ *
+ * Return:
+ * Return the page fragment, otherwise return NULL.
+ */
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
+ * 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.
+ */
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.
+ */
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)
{
--
2.33.0
