[PATCH rfc v2 4/5] page_pool: support page frag API for page pool

[Yunsheng Lin]

Currently each desc use a whole page to do ping-pong page
reusing in most driver. As the page pool has support page
recycling based on elevated refcnt, it makes sense to add
a page frag API in page pool to split a page to different
frag to serve multi descriptions.

This means a huge memory saving for kernel with page size of
64K, as a page can be used by 32 descriptions with 2k buffer
size, comparing to each desc using one page currently.

Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
---
 include/net/page_pool.h | 14 ++++++++++++++
 net/core/page_pool.c    | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 63 insertions(+)

diff --git a/include/net/page_pool.h b/include/net/page_pool.h
index f0e708d..06a5e43 100644
--- a/include/net/page_pool.h
+++ b/include/net/page_pool.h
@@ -80,6 +80,7 @@ struct page_pool_params {
 	enum dma_data_direction dma_dir; /* DMA mapping direction */
 	unsigned int	max_len; /* max DMA sync memory size */
 	unsigned int	offset;  /* DMA addr offset */
+	unsigned int	frag_size;
 };
 
 struct page_pool {
@@ -91,6 +92,8 @@ struct page_pool {
 	unsigned long defer_warn;
 
 	u32 pages_state_hold_cnt;
+	unsigned int frag_offset;
+	struct page *frag_page;
 
 	/*
 	 * Data structure for allocation side
@@ -140,6 +143,17 @@ static inline struct page *page_pool_dev_alloc_pages(struct page_pool *pool)
 	return page_pool_alloc_pages(pool, gfp);
 }
 
+struct page *page_pool_alloc_frag(struct page_pool *pool,
+				  unsigned int *offset, gfp_t gfp);
+
+static inline struct page *page_pool_dev_alloc_frag(struct page_pool *pool,
+						    unsigned int *offset)
+{
+	gfp_t gfp = (GFP_ATOMIC | __GFP_NOWARN);
+
+	return page_pool_alloc_frag(pool, offset, gfp);
+}
+
 /* get the stored dma direction. A driver might decide to treat this locally and
  * avoid the extra cache line from page_pool to determine the direction
  */
diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index a87cbe1..b787033 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -350,6 +350,53 @@ struct page *page_pool_alloc_pages(struct page_pool *pool, gfp_t gfp)
 }
 EXPORT_SYMBOL(page_pool_alloc_pages);
 
+struct page *page_pool_alloc_frag(struct page_pool *pool,
+				  unsigned int *offset, gfp_t gfp)
+{
+	unsigned int frag_offset = pool->frag_offset;
+	unsigned int frag_size = pool->p.frag_size;
+	struct page *frag_page = pool->frag_page;
+	unsigned int max_len = pool->p.max_len;
+
+	if (!frag_page || frag_offset + frag_size > max_len) {
+		frag_page = page_pool_alloc_pages(pool, gfp);
+		if (unlikely(!frag_page)) {
+			pool->frag_page = NULL;
+			return NULL;
+		}
+
+		pool->frag_page = frag_page;
+		frag_offset = 0;
+
+		page_pool_sub_bias(pool, frag_page,
+				   max_len / frag_size - 1);
+	}
+
+	*offset = frag_offset;
+	pool->frag_offset = frag_offset + frag_size;
+
+	return frag_page;
+}
+EXPORT_SYMBOL(page_pool_alloc_frag);
+
+static void page_pool_empty_frag(struct page_pool *pool)
+{
+	unsigned int frag_offset = pool->frag_offset;
+	unsigned int frag_size = pool->p.frag_size;
+	struct page *frag_page = pool->frag_page;
+	unsigned int max_len = pool->p.max_len;
+
+	if (!frag_page)
+		return;
+
+	while (frag_offset + frag_size <= max_len) {
+		page_pool_put_full_page(pool, frag_page, false);
+		frag_offset += frag_size;
+	}
+
+	pool->frag_page = NULL;
+}
+
 /* Calculate distance between two u32 values, valid if distance is below 2^(31)
  *  https://en.wikipedia.org/wiki/Serial_number_arithmetic#General_Solution
  */
@@ -670,6 +717,8 @@ void page_pool_destroy(struct page_pool *pool)
 	if (!page_pool_put(pool))
 		return;
 
+	page_pool_empty_frag(pool);
+
 	if (!page_pool_release(pool))
 		return;
 
-- 
2.7.4