Subject: Re: [PATCH net-next v7 1/6] page_pool: frag API support for 32-bit arch with 64-bit DMA

From: Jakub Kicinski <[email-masked]>
Date: Fri, 18 Aug 2023 14:51:45 -0700

On Fri, 18 Aug 2023 09:12:09 +0300 Ilias Apalodimas wrote:
> > Right, IIUC we don't have enough space to fit dma_addr_t and the
> > refcount, but if we store the dma addr on a shifted u32 instead
> > of using dma_addr_t explicitly - the refcount should fit?  
> 
> struct page looks like this:
> 
> unsigned long dma_addr;
> union {
>       unsigned long dma_addr_upper;
>       atomic_long_t pp_frag_count;
> };

I could be completely misunderstanding the problem.
Let me show you the diff of what I was thinking more or less.

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 5e74ce4a28cd..58ffa8dc745f 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -126,11 +126,6 @@ struct page {
 			unsigned long _pp_mapping_pad;
 			unsigned long dma_addr;
 			union {
-				/**
-				 * dma_addr_upper: might require a 64-bit
-				 * value on 32-bit architectures.
-				 */
-				unsigned long dma_addr_upper;
 				/**
 				 * For frag page support, not supported in
 				 * 32-bit architectures with 64-bit DMA.
diff --git a/include/net/page_pool/helpers.h b/include/net/page_pool/helpers.h
index 94231533a369..6f87a0fa2178 100644
--- a/include/net/page_pool/helpers.h
+++ b/include/net/page_pool/helpers.h
@@ -212,16 +212,24 @@ static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
 	dma_addr_t ret = page->dma_addr;
 
 	if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
-		ret |= (dma_addr_t)page->dma_addr_upper << 16 << 16;
+		ret <<= PAGE_SHIFT;
 
 	return ret;
 }
 
-static inline void page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
+static inline bool page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
 {
+	bool failed = false;
+
 	page->dma_addr = addr;
-	if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
-		page->dma_addr_upper = upper_32_bits(addr);
+	if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT) {
+		page->dma_addr >>= PAGE_SHIFT;
+		/* We assume page alignment to shave off bottom bits,
+		 * if this "compression" doesn't work we need to drop.
+		 */
+		failed = addr != page->dma_addr << PAGE_SHIFT;
+	}
+	return failed;
 }
 
 static inline bool page_pool_put(struct page_pool *pool)
diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index 77cb75e63aca..9ea42e242a89 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -211,10 +211,6 @@ static int page_pool_init(struct page_pool *pool,
 		 */
 	}
 
-	if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT &&
-	    pool->p.flags & PP_FLAG_PAGE_FRAG)
-		return -EINVAL;
-
 #ifdef CONFIG_PAGE_POOL_STATS
 	pool->recycle_stats = alloc_percpu(struct page_pool_recycle_stats);
 	if (!pool->recycle_stats)
@@ -359,12 +355,19 @@ static bool page_pool_dma_map(struct page_pool *pool, struct page *page)
 	if (dma_mapping_error(pool->p.dev, dma))
 		return false;
 
-	page_pool_set_dma_addr(page, dma);
+	if (page_pool_set_dma_addr(page, dma))
+		goto unmap_failed;
 
 	if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
 		page_pool_dma_sync_for_device(pool, page, pool->p.max_len);
 
 	return true;
+
+unmap_failed:
+	dma_unmap_page_attrs(pool->p.dev, dma,
+			     PAGE_SIZE << pool->p.order, pool->p.dma_dir,
+			     DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_WEAK_ORDERING);
+	return false;
 }
 
 static void page_pool_set_pp_info(struct page_pool *pool,