Update documentation about design, implementation and API usages
for page_frag.
CC: Alexander Duyck <alexander.duyck@xxxxxxxxx>
CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
CC: Linux-MM <linux-mm@xxxxxxxxx>
Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
---
Documentation/mm/page_frags.rst | 110 +++++++++++++++++++++++++++++++-
include/linux/page_frag_cache.h | 54 ++++++++++++++++
mm/page_frag_cache.c | 12 +++-
3 files changed, 173 insertions(+), 3 deletions(-)
diff --git a/Documentation/mm/page_frags.rst b/Documentation/mm/page_frags.rst
index 503ca6cdb804..34e654c2956e 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 +42,110 @@ 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
+=============
+
+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.
+
+.. 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
+
+.. kernel-doc:: mm/page_frag_cache.c
+ :identifiers: page_frag_cache_drain page_frag_free
+
+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)
+ goto do_error;
+
+ ...
+
+ page_frag_free(va);
diff --git a/include/linux/page_frag_cache.h b/include/linux/page_frag_cache.h
index 5ae97f93a0a1..a2b1127e8ac8 100644
--- a/include/linux/page_frag_cache.h
+++ b/include/linux/page_frag_cache.h
@@ -28,11 +28,28 @@ static inline bool encoded_page_decode_pfmemalloc(unsigned long encoded_page)
return !!(encoded_page & PAGE_FRAG_CACHE_PFMEMALLOC_BIT);
}
+/**
+ * page_frag_cache_init() - Init page_frag cache.
+ * @nc: page_frag cache from which to init
+ *
+ * Inline helper to initialize the page_frag cache.
+ */
static inline void page_frag_cache_init(struct page_frag_cache *nc)
{
nc->encoded_page = 0;
}
+/**
+ * page_frag_cache_is_pfmemalloc() - Check for pfmemalloc.
+ * @nc: page_frag cache from which to check
+ *
+ * Check if the current page in page_frag cache is allocated from the 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.
+ */
static inline bool page_frag_cache_is_pfmemalloc(struct page_frag_cache *nc)
{
return encoded_page_decode_pfmemalloc(nc->encoded_page);
@@ -57,6 +74,19 @@ static inline unsigned int __page_frag_cache_commit(struct page_frag_cache *nc,
return __page_frag_cache_commit_noref(nc, pfrag, used_sz);
}
+/**
+ * __page_frag_alloc_align() - Allocate 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'
+ *
+ * Allocate a page fragment from page_frag cache with aligning requirement.
+ *
+ * Return:
+ * Virtual address of the page fragment, otherwise return NULL.
+ */
static inline void *__page_frag_alloc_align(struct page_frag_cache *nc,
unsigned int fragsz, gfp_t gfp_mask,
unsigned int align_mask)
@@ -72,6 +102,19 @@ static inline void *__page_frag_alloc_align(struct page_frag_cache *nc,
return va;
}
+/**
+ * page_frag_alloc_align() - Allocate 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 needs to be refilled
+ * @align: the requested aligning requirement for the fragment
+ *
+ * WARN_ON_ONCE() checking for @align before allocating a page fragment from
+ * page_frag cache with aligning requirement.
+ *
+ * Return:
+ * virtual address of the page fragment, otherwise return NULL.
+ */
static inline void *page_frag_alloc_align(struct page_frag_cache *nc,
unsigned int fragsz, gfp_t gfp_mask,
unsigned int align)
@@ -80,6 +123,17 @@ static inline void *page_frag_alloc_align(struct page_frag_cache *nc,
return __page_frag_alloc_align(nc, fragsz, gfp_mask, -align);
}
+/**
+ * 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
+ *
+ * Allocate a page fragment from page_frag cache.
+ *
+ * Return:
+ * virtual address of the page fragment, otherwise return NULL.
+ */
static inline void *page_frag_alloc(struct page_frag_cache *nc,
unsigned int fragsz, gfp_t gfp_mask)
{
diff --git a/mm/page_frag_cache.c b/mm/page_frag_cache.c
index f55d34cf7d43..d014130fb893 100644
--- a/mm/page_frag_cache.c
+++ b/mm/page_frag_cache.c
@@ -70,6 +70,10 @@ static struct page *__page_frag_cache_refill(struct page_frag_cache *nc,
return page;
}
+/**
+ * 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_page)
@@ -182,8 +186,12 @@ void *__page_frag_cache_prepare(struct page_frag_cache *nc, unsigned int fragsz,
}
EXPORT_SYMBOL(__page_frag_cache_prepare);
-/*
- * Frees a page fragment allocated out of either a compound or order 0 page.
+/**
+ * page_frag_free - 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(void *addr)
{
--
2.33.0
