[PATCH rfc v2 2/5] page_pool: add interface for getting and setting pagecnt_bias

[Yunsheng Lin]

As suggested by Alexander, "A DMA mapping should be page
aligned anyway so the lower 12 bits would be reserved 0",
so it might make more sense to repurpose the lower 12 bits
of the dma address to store the pagecnt_bias for elevated
refcnt case in page pool.

As newly added page_pool_get_pagecnt_bias() may be called
outside of the softirq context, so annotate the access to
page->dma_addr[0] with READ_ONCE() and WRITE_ONCE().

Other three interfaces using page->dma_addr[0] is only called
in the softirq context during normal rx processing, hopefully
the barrier in the rx processing will ensure the correct order
between getting and setting pagecnt_bias.

Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
---
 include/net/page_pool.h | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/include/net/page_pool.h b/include/net/page_pool.h
index 8d7744d..5746f17 100644
--- a/include/net/page_pool.h
+++ b/include/net/page_pool.h
@@ -200,7 +200,7 @@ static inline void page_pool_recycle_direct(struct page_pool *pool,
 
 static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
 {
-	dma_addr_t ret = page->dma_addr[0];
+	dma_addr_t ret = READ_ONCE(page->dma_addr[0]) & PAGE_MASK;
 	if (sizeof(dma_addr_t) > sizeof(unsigned long))
 		ret |= (dma_addr_t)page->dma_addr[1] << 16 << 16;
 	return ret;
@@ -208,11 +208,31 @@ static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
 
 static inline void page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
 {
-	page->dma_addr[0] = addr;
+	unsigned long dma_addr_0 = READ_ONCE(page->dma_addr[0]);
+
+	dma_addr_0 &= ~PAGE_MASK;
+	dma_addr_0 |= (addr & PAGE_MASK);
+	WRITE_ONCE(page->dma_addr[0], dma_addr_0);
+
 	if (sizeof(dma_addr_t) > sizeof(unsigned long))
 		page->dma_addr[1] = upper_32_bits(addr);
 }
 
+static inline int page_pool_get_pagecnt_bias(struct page *page)
+{
+	return (READ_ONCE(page->dma_addr[0]) & ~PAGE_MASK);
+}
+
+static inline void page_pool_set_pagecnt_bias(struct page *page, int bias)
+{
+	unsigned long dma_addr_0 = READ_ONCE(page->dma_addr[0]);
+
+	dma_addr_0 &= PAGE_MASK;
+	dma_addr_0 |= (bias & ~PAGE_MASK);
+
+	WRITE_ONCE(page->dma_addr[0], dma_addr_0);
+}
+
 static inline bool is_page_pool_compiled_in(void)
 {
 #ifdef CONFIG_PAGE_POOL
-- 
2.7.4