netmem: replace __netmem_clear_lsb() with netmem_to_nmdesc()

Now that we have struct netmem_desc, it'd better access the pp fields
via struct netmem_desc rather than struct net_iov.

Introduce netmem_to_nmdesc() for safely converting netmem_ref to
netmem_desc regardless of the type underneath e.i. netmem_desc, net_iov.

While at it, remove __netmem_clear_lsb() and make netmem_to_nmdesc()
used instead.

Suggested-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Byungchul Park <byungchul@sk.com>
Reviewed-by: Mina Almasry <almasrymina@google.com>
Link: https://patch.msgid.link/20251013044133.69472-1-byungchul@sk.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

include/net/netmem.h

@@ -247,6 +247,23 @@ static inline unsigned long netmem_pfn_trace(netmem_ref netmem)
 	return page_to_pfn(netmem_to_page(netmem));
 }
 
+/* XXX: How to extract netmem_desc from page must be changed, once
+ * netmem_desc no longer overlays on page and will be allocated through
+ * slab.
+ */
+#define __pp_page_to_nmdesc(p)	(_Generic((p),				\
+	const struct page * :	(const struct netmem_desc *)(p),	\
+	struct page * :		(struct netmem_desc *)(p)))
+
+/* CAUTION: Check if the page is a pp page before calling this helper or
+ * know it's a pp page.
+ */
+#define pp_page_to_nmdesc(p)						\
+({									\
+	DEBUG_NET_WARN_ON_ONCE(!page_pool_page_is_pp(p));		\
+	__pp_page_to_nmdesc(p);						\
+})
+
 /**
  * __netmem_to_nmdesc - unsafely get pointer to the &netmem_desc backing
  * @netmem
@@ -265,43 +282,26 @@ static inline struct netmem_desc *__netmem_to_nmdesc(netmem_ref netmem)
 	return (__force struct netmem_desc *)netmem;
 }
 
-/* __netmem_clear_lsb - convert netmem_ref to struct net_iov * for access to
+/* netmem_to_nmdesc - convert netmem_ref to struct netmem_desc * for
- * common fields.
+ * access to common fields.
- * @netmem: netmem reference to extract as net_iov.
+ * @netmem: netmem reference to get netmem_desc.
  *
- * All the sub types of netmem_ref (page, net_iov) have the same pp, pp_magic,
+ * All the sub types of netmem_ref (netmem_desc, net_iov) have the same
- * dma_addr, and pp_ref_count fields at the same offsets. Thus, we can access
+ * pp, pp_magic, dma_addr, and pp_ref_count fields via netmem_desc.
- * these fields without a type check to make sure that the underlying mem is
- * net_iov or page.
  *
- * The resulting value of this function can only be used to access the fields
+ * Return: the pointer to struct netmem_desc * regardless of its
- * that are NET_IOV_ASSERT_OFFSET'd. Accessing any other fields will result in
+ * underlying type.
- * undefined behavior.
- *
- * Return: the netmem_ref cast to net_iov* regardless of its underlying type.
  */
-static inline struct net_iov *__netmem_clear_lsb(netmem_ref netmem)
+static inline struct netmem_desc *netmem_to_nmdesc(netmem_ref netmem)
 {
-	return (struct net_iov *)((__force unsigned long)netmem & ~NET_IOV);
+	void *p = (void *)((__force unsigned long)netmem & ~NET_IOV);
+
+	if (netmem_is_net_iov(netmem))
+		return &((struct net_iov *)p)->desc;
+
+	return __pp_page_to_nmdesc((struct page *)p);
 }
 
-/* XXX: How to extract netmem_desc from page must be changed, once
- * netmem_desc no longer overlays on page and will be allocated through
- * slab.
- */
-#define __pp_page_to_nmdesc(p)	(_Generic((p),				\
-	const struct page * :	(const struct netmem_desc *)(p),	\
-	struct page * :		(struct netmem_desc *)(p)))
-
-/* CAUTION: Check if the page is a pp page before calling this helper or
- * know it's a pp page.
- */
-#define pp_page_to_nmdesc(p)						\
-({									\
-	DEBUG_NET_WARN_ON_ONCE(!page_pool_page_is_pp(p));		\
-	__pp_page_to_nmdesc(p);						\
-})
-
 /**
  * __netmem_get_pp - unsafely get pointer to the &page_pool backing @netmem
  * @netmem: netmem reference to get the pointer from
@@ -320,12 +320,12 @@ static inline struct page_pool *__netmem_get_pp(netmem_ref netmem)
 
 static inline struct page_pool *netmem_get_pp(netmem_ref netmem)
 {
-	return __netmem_clear_lsb(netmem)->pp;
+	return netmem_to_nmdesc(netmem)->pp;
 }
 
 static inline atomic_long_t *netmem_get_pp_ref_count_ref(netmem_ref netmem)
 {
-	return &__netmem_clear_lsb(netmem)->pp_ref_count;
+	return &netmem_to_nmdesc(netmem)->pp_ref_count;
 }
 
 static inline bool netmem_is_pref_nid(netmem_ref netmem, int pref_nid)
@@ -390,7 +390,7 @@ static inline bool netmem_is_pfmemalloc(netmem_ref netmem)
 
 static inline unsigned long netmem_get_dma_addr(netmem_ref netmem)
 {
-	return __netmem_clear_lsb(netmem)->dma_addr;
+	return netmem_to_nmdesc(netmem)->dma_addr;
 }
 
 void get_netmem(netmem_ref netmem);

net/core/netmem_priv.h

@@ -5,19 +5,19 @@
 
 static inline unsigned long netmem_get_pp_magic(netmem_ref netmem)
 {
-	return __netmem_clear_lsb(netmem)->pp_magic & ~PP_DMA_INDEX_MASK;
+	return netmem_to_nmdesc(netmem)->pp_magic & ~PP_DMA_INDEX_MASK;
 }
 
 static inline void netmem_or_pp_magic(netmem_ref netmem, unsigned long pp_magic)
 {
-	__netmem_clear_lsb(netmem)->pp_magic |= pp_magic;
+	netmem_to_nmdesc(netmem)->pp_magic |= pp_magic;
 }
 
 static inline void netmem_clear_pp_magic(netmem_ref netmem)
 {
-	WARN_ON_ONCE(__netmem_clear_lsb(netmem)->pp_magic & PP_DMA_INDEX_MASK);
+	WARN_ON_ONCE(netmem_to_nmdesc(netmem)->pp_magic & PP_DMA_INDEX_MASK);
 
-	__netmem_clear_lsb(netmem)->pp_magic = 0;
+	netmem_to_nmdesc(netmem)->pp_magic = 0;
 }
 
 static inline bool netmem_is_pp(netmem_ref netmem)
@@ -27,13 +27,13 @@ static inline bool netmem_is_pp(netmem_ref netmem)
 
 static inline void netmem_set_pp(netmem_ref netmem, struct page_pool *pool)
 {
-	__netmem_clear_lsb(netmem)->pp = pool;
+	netmem_to_nmdesc(netmem)->pp = pool;
 }
 
 static inline void netmem_set_dma_addr(netmem_ref netmem,
 				       unsigned long dma_addr)
 {
-	__netmem_clear_lsb(netmem)->dma_addr = dma_addr;
+	netmem_to_nmdesc(netmem)->dma_addr = dma_addr;
 }
 
 static inline unsigned long netmem_get_dma_index(netmem_ref netmem)
@@ -43,7 +43,7 @@ static inline unsigned long netmem_get_dma_index(netmem_ref netmem)
 	if (WARN_ON_ONCE(netmem_is_net_iov(netmem)))
 		return 0;
 
-	magic = __netmem_clear_lsb(netmem)->pp_magic;
+	magic = netmem_to_nmdesc(netmem)->pp_magic;
 
 	return (magic & PP_DMA_INDEX_MASK) >> PP_DMA_INDEX_SHIFT;
 }
@@ -57,6 +57,6 @@ static inline void netmem_set_dma_index(netmem_ref netmem,
 		return;
 
 	magic = netmem_get_pp_magic(netmem) | (id << PP_DMA_INDEX_SHIFT);
-	__netmem_clear_lsb(netmem)->pp_magic = magic;
+	netmem_to_nmdesc(netmem)->pp_magic = magic;
 }
 #endif