[RFC net-next v2] netmem: replace __netmem_clear_lsb() with netmem_to_nmdesc()

[RFC net-next v2] netmem: replace __netmem_clear_lsb() with netmem_to_nmdesc()

[Byungchul Park]

Changes from RFC:
	1. Optimize the implementation of netmem_to_nmdesc to use less
	   instructions (feedbacked by Pavel)

---8<---
From 6a0dbaecbf9a2425afe73565914eaa762c5d15c8 Mon Sep 17 00:00:00 2001
From: Byungchul Park <byungchul@sk.com>
Date: Tue, 29 Jul 2025 19:34:12 +0900
Subject: [RFC net-next v2] 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>
---
 include/net/netmem.h   | 66 +++++++++++++++++++++---------------------
 net/core/netmem_priv.h | 16 +++++-----
 2 files changed, 41 insertions(+), 41 deletions(-)

diff --git a/include/net/netmem.h b/include/net/netmem.h
index f7dacc9e75fd..651e2c62d1dd 100644
--- a/include/net/netmem.h
+++ b/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
@@ -265,42 +282,25 @@ 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
- * 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.
+/* netmem_to_nmdesc - convert netmem_ref to struct netmem_desc * for
+ * access to common fields.
+ * @netmem: netmem reference to get netmem_desc.
  *
- * 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.
+ * All the sub types of netmem_ref (netmem_desc, net_iov) have the same
+ * pp, pp_magic, dma_addr, and pp_ref_count fields via netmem_desc.
  *
- * Return: the netmem_ref cast to net_iov* regardless of its underlying type.
+ * Return: the pointer to struct netmem_desc * 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);
 
-/* 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)))
+	if (netmem_is_net_iov(netmem))
+		return &((struct net_iov *)p)->desc;
 
-/* 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);						\
-})
+	return __pp_page_to_nmdesc((struct page *)p);
+}
 
 /**
  * __netmem_get_pp - unsafely get pointer to the &page_pool backing @netmem
@@ -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);
diff --git a/net/core/netmem_priv.h b/net/core/netmem_priv.h
index cd95394399b4..23175cb2bd86 100644
--- a/net/core/netmem_priv.h
+++ b/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

base-commit: fa582ca7e187a15e772e6a72fe035f649b387a60
-- 
2.17.1

Re: [RFC net-next v2] netmem: replace __netmem_clear_lsb() with netmem_to_nmdesc()

[Mina Almasry]

On Tue, Jul 29, 2025 at 3:42 AM Byungchul Park <byungchul@sk.com> wrote:
>
> Changes from RFC:
>         1. Optimize the implementation of netmem_to_nmdesc to use less
>            instructions (feedbacked by Pavel)
>
> ---8<---
> From 6a0dbaecbf9a2425afe73565914eaa762c5d15c8 Mon Sep 17 00:00:00 2001
> From: Byungchul Park <byungchul@sk.com>
> Date: Tue, 29 Jul 2025 19:34:12 +0900
> Subject: [RFC net-next v2] 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>

Thanks for cleaning this up. This bit of tech debt was bothering me!

-- 
Thanks,
Mina

Re: [RFC net-next v2] netmem: replace __netmem_clear_lsb() with netmem_to_nmdesc()

[Pavel Begunkov]

On 7/29/25 11:41, Byungchul Park wrote:
> Changes from RFC:
> 	1. Optimize the implementation of netmem_to_nmdesc to use less
> 	   instructions (feedbacked by Pavel)
> 
> ---8<---
>  From 6a0dbaecbf9a2425afe73565914eaa762c5d15c8 Mon Sep 17 00:00:00 2001
> From: Byungchul Park <byungchul@sk.com>
> Date: Tue, 29 Jul 2025 19:34:12 +0900
> Subject: [RFC net-next v2] 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.

I'll ultimately need this in another tree as indicated in the
original diff, so I'll take it into a branch and send it out
with other patches.

-- 
Pavel Begunkov

Re: [RFC net-next v2] netmem: replace __netmem_clear_lsb() with netmem_to_nmdesc()

[Byungchul Park]

On Sun, Aug 10, 2025 at 08:39:42PM +0100, Pavel Begunkov wrote:
> On 7/29/25 11:41, Byungchul Park wrote:
> > Changes from RFC:
> >       1. Optimize the implementation of netmem_to_nmdesc to use less
> >          instructions (feedbacked by Pavel)
> > 
> > ---8<---
> >  From 6a0dbaecbf9a2425afe73565914eaa762c5d15c8 Mon Sep 17 00:00:00 2001
> > From: Byungchul Park <byungchul@sk.com>
> > Date: Tue, 29 Jul 2025 19:34:12 +0900
> > Subject: [RFC net-next v2] 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.
> 
> I'll ultimately need this in another tree as indicated in the
> original diff, so I'll take it into a branch and send it out

Just curious.  What is the original diff?

	Byungchul

> with other patches.
> 
> --
> Pavel Begunkov

Re: [RFC net-next v2] netmem: replace __netmem_clear_lsb() with netmem_to_nmdesc()

[Pavel Begunkov]

On 8/11/25 05:23, Byungchul Park wrote:
> On Sun, Aug 10, 2025 at 08:39:42PM +0100, Pavel Begunkov wrote:
>> On 7/29/25 11:41, Byungchul Park wrote:
>>> Changes from RFC:
>>>        1. Optimize the implementation of netmem_to_nmdesc to use less
>>>           instructions (feedbacked by Pavel)
>>>
>>> ---8<---
>>>   From 6a0dbaecbf9a2425afe73565914eaa762c5d15c8 Mon Sep 17 00:00:00 2001
>>> From: Byungchul Park <byungchul@sk.com>
>>> Date: Tue, 29 Jul 2025 19:34:12 +0900
>>> Subject: [RFC net-next v2] 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.
>>
>> I'll ultimately need this in another tree as indicated in the
>> original diff, so I'll take it into a branch and send it out
> 
> Just curious.  What is the original diff?

It was this one:

https://lore.kernel.org/all/a7bd1e6f-b854-4172-a29a-3f0662c6fd6e@gmail.com/

-- 
Pavel Begunkov