Message-ID: <20230912083126.65484-6-linyunsheng@huawei.com>
Date: Tue, 12 Sep 2023 16:31:24 +0800
From: Yunsheng Lin <linyunsheng@...wei.com>
To: <davem@...emloft.net>, <kuba@...nel.org>, <pabeni@...hat.com>
CC: <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>, Yunsheng Lin
	<linyunsheng@...wei.com>, Lorenzo Bianconi <lorenzo@...nel.org>, Alexander
 Duyck <alexander.duyck@...il.com>, Liang Chen <liangchen.linux@...il.com>,
	Alexander Lobakin <aleksander.lobakin@...el.com>, Jesper Dangaard Brouer
	<hawk@...nel.org>, Ilias Apalodimas <ilias.apalodimas@...aro.org>, Eric
 Dumazet <edumazet@...gle.com>, Jonathan Corbet <corbet@....net>, Alexei
 Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, John
 Fastabend <john.fastabend@...il.com>, <linux-doc@...r.kernel.org>,
	<bpf@...r.kernel.org>
Subject: [PATCH net-next v8 5/6] page_pool: update document about frag API

As more drivers begin to use the frag API, update the
document about how to decide which API to use for the
driver author.

Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
CC: Lorenzo Bianconi <lorenzo@...nel.org>
CC: Alexander Duyck <alexander.duyck@...il.com>
CC: Liang Chen <liangchen.linux@...il.com>
CC: Alexander Lobakin <aleksander.lobakin@...el.com>
---
 Documentation/networking/page_pool.rst |  4 +-
 include/net/page_pool/helpers.h        | 88 ++++++++++++++++++++++----
 2 files changed, 79 insertions(+), 13 deletions(-)

diff --git a/Documentation/networking/page_pool.rst b/Documentation/networking/page_pool.rst
index 215ebc92752c..0c0705994f51 100644
--- a/Documentation/networking/page_pool.rst
+++ b/Documentation/networking/page_pool.rst
@@ -58,7 +58,9 @@ a page will cause no race conditions is enough.
 
 .. kernel-doc:: include/net/page_pool/helpers.h
    :identifiers: page_pool_put_page page_pool_put_full_page
-		 page_pool_recycle_direct page_pool_dev_alloc_pages
+		 page_pool_recycle_direct page_pool_cache_free
+		 page_pool_dev_alloc_pages page_pool_dev_alloc_frag
+		 page_pool_dev_alloc page_pool_dev_cache_alloc
 		 page_pool_get_dma_addr page_pool_get_dma_dir
 
 .. kernel-doc:: net/core/page_pool.c
diff --git a/include/net/page_pool/helpers.h b/include/net/page_pool/helpers.h
index c0e6c7d1b219..b20afe22c17d 100644
--- a/include/net/page_pool/helpers.h
+++ b/include/net/page_pool/helpers.h
@@ -8,23 +8,47 @@
 /**
  * DOC: page_pool allocator
  *
- * The page_pool allocator is optimized for the XDP mode that
- * uses one frame per-page, but it can fallback on the
- * regular page allocator APIs.
+ * The page_pool allocator is optimized for recycling page or page frag used by
+ * skb packet and xdp frame.
  *
- * Basic use involves replacing alloc_pages() calls with the
- * page_pool_alloc_pages() call.  Drivers should use
- * page_pool_dev_alloc_pages() replacing dev_alloc_pages().
+ * Basic use involves replacing napi_alloc_frag() and alloc_pages() calls with
+ * page_pool_cache_alloc() and page_pool_alloc(), which allocate memory with or
+ * without page splitting depending on the requested memory size.
  *
- * API keeps track of in-flight pages, in order to let API user know
- * when it is safe to free a page_pool object.  Thus, API users
- * must call page_pool_put_page() to free the page, or attach
- * the page to a page_pool-aware objects like skbs marked with
+ * If the driver knows that it always requires full pages or its allocations are
+ * always smaller than half a page, it can use one of the more specific API
+ * calls:
+ *
+ * 1. page_pool_alloc_pages(): allocate memory without page splitting when
+ * driver knows that the memory it need is always bigger than half of the page
+ * allocated from page pool. There is no cache line dirtying for 'struct page'
+ * when a page is recycled back to the page pool.
+ *
+ * 2. page_pool_alloc_frag(): allocate memory with page splitting when driver
+ * knows that the memory it need is always smaller than or equal to half of the
+ * page allocated from page pool. Page splitting enables memory saving and thus
+ * avoids TLB/cache miss for data access, but there also is some cost to
+ * implement page splitting, mainly some cache line dirtying/bouncing for
+ * 'struct page' and atomic operation for page->pp_frag_count.
+ *
+ * API keeps track of in-flight pages, in order to let API user know when it is
+ * safe to free a page_pool object, the API users must call page_pool_put_page()
+ * or page_pool_cache_free() to free the pp page or the pp buffer, or attach the
+ * pp page or the pp buffer to a page_pool-aware objects like skbs marked with
  * skb_mark_for_recycle().
  *
- * API user must call page_pool_put_page() once on a page, as it
- * will either recycle the page, or in case of refcnt > 1, it will
+ * page_pool_put_page() may be called multi times on the same page if a page is
+ * split into multi frags. For the last frag, see page_pool_is_last_frag(), it
+ * will either recycle the page, or in case of page->_refcount > 1, it will
  * release the DMA mapping and in-flight state accounting.
+ *
+ * dma_sync_single_range_for_device() is only called for the last pp page user
+ * when page_pool is created with PP_FLAG_DMA_SYNC_DEV flag, so it depend on the
+ * last freed frag to do the sync_for_device operation for all frags in the same
+ * page when a page is split, the API user must setup pool->p.max_len and
+ * pool->p.offset correctly and ensure that page_pool_put_page() is called with
+ * dma_sync_size being -1 for page_pool_alloc(), page_pool_cache_alloc() and
+ * page_pool_alloc_frag() API.
  */
 #ifndef _NET_PAGE_POOL_HELPERS_H
 #define _NET_PAGE_POOL_HELPERS_H
@@ -73,6 +97,17 @@ static inline struct page *page_pool_dev_alloc_pages(struct page_pool *pool)
 	return page_pool_alloc_pages(pool, gfp);
 }
 
+/**
+ * page_pool_dev_alloc_frag() - allocate a page frag.
+ * @pool: pool from which to allocate
+ * @offset: offset to the allocated page
+ * @size: requested size
+ *
+ * Get a page frag from the page allocator or page_pool caches.
+ *
+ * Return:
+ * Returns allocated page frag, otherwise return NULL.
+ */
 static inline struct page *page_pool_dev_alloc_frag(struct page_pool *pool,
 						    unsigned int *offset,
 						    unsigned int size)
@@ -111,6 +146,17 @@ static inline struct page *page_pool_alloc(struct page_pool *pool,
 	return page;
 }
 
+/**
+ * page_pool_dev_alloc() - allocate a page or a page frag.
+ * @pool: pool from which to allocate
+ * @offset: offset to the allocated page
+ * @size: in as the requested size, out as the allocated size
+ *
+ * Get a page or a page frag from the page allocator or page_pool caches.
+ *
+ * Return:
+ * Returns a page or a page frag, otherwise return NULL.
+ */
 static inline struct page *page_pool_dev_alloc(struct page_pool *pool,
 					       unsigned int *offset,
 					       unsigned int *size)
@@ -133,6 +179,16 @@ static inline void *page_pool_cache_alloc(struct page_pool *pool,
 	return page_address(page) + offset;
 }
 
+/**
+ * page_pool_dev_cache_alloc() - allocate a cache.
+ * @pool: pool from which to allocate
+ * @size: in as the requested size, out as the allocated size
+ *
+ * Get a cache from the page allocator or page_pool caches.
+ *
+ * Return:
+ * Returns the addr for the allocated cache, otherwise return NULL.
+ */
 static inline void *page_pool_dev_cache_alloc(struct page_pool *pool,
 					      unsigned int *size)
 {
@@ -281,6 +337,14 @@ static inline void page_pool_recycle_direct(struct page_pool *pool,
 #define PAGE_POOL_32BIT_ARCH_WITH_64BIT_DMA	\
 		(sizeof(dma_addr_t) > sizeof(unsigned long))
 
+/**
+ * page_pool_cache_free() - free a cache into the page_pool
+ * @pool: pool from which cache was allocated
+ * @data: addr of cache to be free
+ * @allow_direct: freed by the consumer, allow lockless caching
+ *
+ * Free a cache allocated from page_pool_dev_cache_alloc().
+ */
 static inline void page_pool_cache_free(struct page_pool *pool, void *data,
 					bool allow_direct)
 {
-- 
2.33.0

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