[PATCH net-next v2 1/3] netmem: fix netmem comments

[Mina Almasry <almasrymina@google.com>]

Trivial fix to a couple of outdated netmem comments. No code changes,
just more accurately describing current code.

Signed-off-by: Mina Almasry <almasrymina@google.com>

---

v2: https://lore.kernel.org/netdev/20250613042804.3259045-2-almasrymina@google.com/
- Adjust comment for clearing lsb as (Jakub)
---
 include/net/netmem.h | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/include/net/netmem.h b/include/net/netmem.h
index 386164fb9c18..850869b45b45 100644
--- a/include/net/netmem.h
+++ b/include/net/netmem.h
@@ -89,8 +89,7 @@ static inline unsigned int net_iov_idx(const struct net_iov *niov)
  * typedef netmem_ref - a nonexistent type marking a reference to generic
  * network memory.
  *
- * A netmem_ref currently is always a reference to a struct page. This
- * abstraction is introduced so support for new memory types can be added.
+ * A netmem_ref can be a struct page* or a struct net_iov* underneath.
  *
  * Use the supplied helpers to obtain the underlying memory pointer and fields.
  */
@@ -117,9 +116,6 @@ static inline struct page *__netmem_to_page(netmem_ref netmem)
 	return (__force struct page *)netmem;
 }
 
-/* This conversion fails (returns NULL) if the netmem_ref is not struct page
- * backed.
- */
 static inline struct page *netmem_to_page(netmem_ref netmem)
 {
 	if (WARN_ON_ONCE(netmem_is_net_iov(netmem)))
@@ -178,6 +174,21 @@ static inline unsigned long netmem_pfn_trace(netmem_ref netmem)
 	return page_to_pfn(netmem_to_page(netmem));
 }
 
+/* __netmem_clear_lsb - convert netmem_ref to struct net_iov * for access to
+ * common fields.
+ * @netmem: netmem reference to extract as net_iov.
+ *
+ * All the sub types of netmem_ref (page, net_iov) have the same pp, pp_magic,
+ * dma_addr, and pp_ref_count fields at the same offsets. Thus, we can access
+ * 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
+ * that are NET_IOV_ASSERT_OFFSET'd. Accessing any other fields will result in
+ * 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)
 {
 	return (struct net_iov *)((__force unsigned long)netmem & ~NET_IOV);

base-commit: 8909f5f4ecd551c2299b28e05254b77424c8c7dc
-- 
2.50.0.rc1.591.g9c95f17f64-goog