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 | 148 +++++++++++++++++++++++++++++++-
include/linux/page_frag_cache.h | 133 ++++++++++++++++++++++++++++
mm/page_frag_cache.c | 4 +
3 files changed, 284 insertions(+), 1 deletion(-)
diff --git a/Documentation/mm/page_frags.rst b/Documentation/mm/page_frags.rst
index 503ca6cdb804..ac9dd9e8ee16 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 +42,148 @@ 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, the allocation side does not
+allow concurrent calling, it is assumed that the caller must ensure there is not
+concurrent alloc calling to the same page_frag_cache instance by using it's own
+lock or rely on some lockless guarantee like NAPI softirq.
+
+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 can do better if there is more memory available. Introduce
+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, caller need to report back to the page_frag core
+how much memory it actually use by calling commit API, or not calling the commit
+API 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_va __page_frag_alloc_va_align
+ page_frag_alloc_va_align page_frag_alloc_va_prepare
+ page_frag_alloc_va_prepare_align page_frag_alloc_pg_prepare
+ page_frag_alloc_prepare page_frag_alloc_commit
+ page_frag_alloc_commit_noref page_frag_free_va
+
+.. kernel-doc:: mm/page_frag_cache.c
+ :identifiers: page_frag_cache_drain
+
+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 28185969cd2c..529e7c040dad 100644
--- a/include/linux/page_frag_cache.h
+++ b/include/linux/page_frag_cache.h
@@ -31,11 +31,28 @@ 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)
{
nc->va = NULL;
}
+/**
+ * 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 !!nc->pfmemalloc;
@@ -46,6 +63,17 @@ void __page_frag_cache_drain(struct page *page, unsigned int count);
void *page_frag_cache_refill(struct page_frag_cache *nc, unsigned int fragsz,
gfp_t gfp_mask);
+/**
+ * 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)
{
@@ -63,6 +91,19 @@ static inline void *page_frag_alloc_va(struct page_frag_cache *nc,
return va + offset;
}
+/**
+ * __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
+ *
+ * Get a page fragment from page_frag cache with aligning requirement.
+ *
+ * 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,
@@ -75,6 +116,19 @@ static inline void *__page_frag_alloc_va_align(struct page_frag_cache *nc,
return page_frag_alloc_va(nc, fragsz, gfp_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
+ *
+ * WARN_ON_ONCE() checking for align and fragsz before getting a page fragment
+ * from page_frag cache with aligning requirement.
+ *
+ * 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,
@@ -86,6 +140,19 @@ 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_alloc_va_prepare() - Prepare allocing a page fragment.
+ * @nc: page_frag cache from which to prepare
+ * @offset: out as the offset of the page fragment
+ * @size: in as the requested size, out as the available size
+ * @gfp_mask: the allocation gfp to use when cache need to be refilled
+ *
+ * Prepare a page fragment with minimum size of ‘size’, 'size' 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(struct page_frag_cache *nc,
unsigned int *offset,
unsigned int *size,
@@ -108,6 +175,21 @@ static inline void *page_frag_alloc_va_prepare(struct page_frag_cache *nc,
return va + *offset;
}
+/**
+ * page_frag_alloc_va_prepare_align() - Prepare allocing a page fragment with
+ * aligning requirement.
+ * @nc: page_frag cache from which to prepare
+ * @offset: out as the offset of the page fragment
+ * @size: in as the requested size, out as the available size
+ * @align: the requested aligning requirement
+ * @gfp_mask: the allocation gfp to use when cache need to be refilled
+ *
+ * Prepare an aligned page fragment with minimum size of ‘size’, 'size' 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 *offset,
unsigned int *size,
@@ -144,6 +226,19 @@ static inline void *__page_frag_alloc_pg_prepare(struct page_frag_cache *nc,
return va;
}
+/**
+ * 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
+ * @size: 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 ‘size’, 'size' is also used to
+ * report the maximum size of the page fragment the caller can use.
+ *
+ * Return:
+ * Return the page fragment, otherwise return NULL.
+ */
#define page_frag_alloc_pg_prepare(nc, offset, size, gfp) \
({ \
struct page *__page = NULL; \
@@ -179,6 +274,21 @@ static inline void *__page_frag_alloc_prepare(struct page_frag_cache *nc,
return nc_va;
}
+/**
+ * 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
+ * @size: in as the requested size, out as the available size
+ * @va: out as the va 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 ‘size’, 'size' 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.
+ */
#define page_frag_alloc_prepare(nc, offset, size, va, gfp) \
({ \
struct page *__page = NULL; \
@@ -191,6 +301,14 @@ static inline void *__page_frag_alloc_prepare(struct page_frag_cache *nc,
__page; \
})
+/**
+ * page_frag_alloc_commit - Commit allocing a page fragment.
+ * @nc: page_frag cache from which to commit
+ * @offset: offset of the page fragment
+ * @size: size of the page fragment has been used
+ *
+ * Commit the alloc preparing by passing offset and the actual used size.
+ */
static inline void page_frag_alloc_commit(struct page_frag_cache *nc,
unsigned int offset,
unsigned int size)
@@ -199,6 +317,17 @@ static inline void page_frag_alloc_commit(struct page_frag_cache *nc,
nc->offset = offset + size;
}
+/**
+ * page_frag_alloc_commit_noref - Commit allocing a page fragment without taking
+ * page refcount.
+ * @nc: page_frag cache from which to commit
+ * @offset: offset of the page fragment
+ * @size: size of the page fragment has been used
+ *
+ * Commit the alloc preparing by passing offset and 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 offset,
unsigned int size)
@@ -206,6 +335,10 @@ static inline void page_frag_alloc_commit_noref(struct page_frag_cache *nc,
nc->offset = offset + size;
}
+/**
+ * page_frag_free_va - Free a page fragment by va.
+ * @addr: va of page fragment to be freed
+ */
void page_frag_free_va(void *addr);
#endif
diff --git a/mm/page_frag_cache.c b/mm/page_frag_cache.c
index 8774cb07e630..8b1d35aafcc1 100644
--- a/mm/page_frag_cache.c
+++ b/mm/page_frag_cache.c
@@ -60,6 +60,10 @@ static bool __page_frag_cache_refill(struct page_frag_cache *nc,
return true;
}
+/**
+ * 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->va)
--
2.33.0
