Re: [PATCH net-next v9 1/8] netmem: introduce struct netmem_desc mirroring struct page

[Pavel Begunkov <asml.silence@gmail.com>]

On 7/14/25 05:23, Byungchul Park wrote:
> On Sat, Jul 12, 2025 at 03:39:59PM +0100, Pavel Begunkov wrote:
>> On 7/10/25 09:28, Byungchul Park wrote:
>>> To simplify struct page, the page pool members of struct page should be
>>> moved to other, allowing these members to be removed from struct page.
>>>
>>> Introduce a network memory descriptor to store the members, struct
>>> netmem_desc, and make it union'ed with the existing fields in struct
>>> net_iov, allowing to organize the fields of struct net_iov.
>>
>> FWIW, regardless of memdesc business, I think it'd be great to have
>> this patch, as it'll help with some of the netmem casting ugliness and
>> shed some cycles as well. For example, we have a bunch of
>> niov -> netmem -> niov casts in various places.
> 
> If Jakub agrees with this, I will re-post this as a separate patch so
> that works that require this base can go ahead.

I think it'd be a good idea. It's needed to clean up netmem handling,
and I'll convert io_uring and get rid of the union in niov.

The diff below should give a rough idea of what I want to use it for.
It kills __netmem_clear_lsb() to avoid casting struct page * to niov.
And saves some masking for zcrx, see page_pool_get_dma_addr_nmdesc(),
and there are more places like that.


diff --git a/include/net/netmem.h b/include/net/netmem.h
index 535cf17b9134..41f3a3fd6b6c 100644
--- a/include/net/netmem.h
+++ b/include/net/netmem.h
@@ -247,6 +247,8 @@ static inline unsigned long netmem_pfn_trace(netmem_ref netmem)
  	return page_to_pfn(netmem_to_page(netmem));
  }
  
+#define pp_page_to_nmdesc(page)	((struct netmem_desc *)(page))
+
  /* __netmem_clear_lsb - convert netmem_ref to struct net_iov * for access to
   * common fields.
   * @netmem: netmem reference to extract as net_iov.
@@ -262,11 +264,18 @@ static inline unsigned long netmem_pfn_trace(netmem_ref netmem)
   *
   * 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 net_iov *__netmem_to_niov(netmem_ref netmem)
  {
  	return (struct net_iov *)((__force unsigned long)netmem & ~NET_IOV);
  }
  
+static inline struct netmem_desc *netmem_to_nmdesc(netmem_ref netmem)
+{
+	if (netmem_is_net_iov(netmem))
+		return &__netmem_to_niov(netmem)->desc;
+	return pp_page_to_nmdesc(__netmem_to_page(netmem));
+}
+
  /**
   * __netmem_get_pp - unsafely get pointer to the &page_pool backing @netmem
   * @netmem: netmem reference to get the pointer from
@@ -280,17 +289,17 @@ static inline struct net_iov *__netmem_clear_lsb(netmem_ref netmem)
   */
  static inline struct page_pool *__netmem_get_pp(netmem_ref netmem)
  {
-	return __netmem_to_page(netmem)->pp;
+	return pp_page_to_nmdesc(__netmem_to_page(netmem))->pp;
  }
  
  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)
@@ -355,7 +364,7 @@ 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/include/net/page_pool/helpers.h b/include/net/page_pool/helpers.h
index db180626be06..002858f3bcb3 100644
--- a/include/net/page_pool/helpers.h
+++ b/include/net/page_pool/helpers.h
@@ -425,9 +425,9 @@ static inline void page_pool_free_va(struct page_pool *pool, void *va,
  	page_pool_put_page(pool, virt_to_head_page(va), -1, allow_direct);
  }
  
-static inline dma_addr_t page_pool_get_dma_addr_netmem(netmem_ref netmem)
+static inline dma_addr_t page_pool_get_dma_addr_nmdesc(struct netmem_desc *desc)
  {
-	dma_addr_t ret = netmem_get_dma_addr(netmem);
+	dma_addr_t ret = desc->dma_addr;
  
  	if (PAGE_POOL_32BIT_ARCH_WITH_64BIT_DMA)
  		ret <<= PAGE_SHIFT;
@@ -435,6 +435,13 @@ static inline dma_addr_t page_pool_get_dma_addr_netmem(netmem_ref netmem)
  	return ret;
  }
  
+static inline dma_addr_t page_pool_get_dma_addr_netmem(netmem_ref netmem)
+{
+	struct netmem_desc *desc = netmem_to_nmdesc(netmem);
+
+	return page_pool_get_dma_addr_nmdesc(desc);
+}
+
  /**
   * page_pool_get_dma_addr() - Retrieve the stored DMA address.
   * @page:	page allocated from a page pool
diff --git a/io_uring/zcrx.c b/io_uring/zcrx.c
index 085eeed8cd50..2e80692d9ee1 100644
--- a/io_uring/zcrx.c
+++ b/io_uring/zcrx.c
@@ -290,7 +290,7 @@ static void io_zcrx_sync_for_device(const struct page_pool *pool,
  	if (!dma_dev_need_sync(pool->p.dev))
  		return;
  
-	dma_addr = page_pool_get_dma_addr_netmem(net_iov_to_netmem(niov));
+	dma_addr = page_pool_get_dma_addr_nmdesc(&niov->desc);
  	__dma_sync_single_for_device(pool->p.dev, dma_addr + pool->p.offset,
  				     PAGE_SIZE, pool->p.dma_dir);
  #endif
diff --git a/net/core/netmem_priv.h b/net/core/netmem_priv.h
index cd95394399b4..97d4beda9174 100644
--- a/net/core/netmem_priv.h
+++ b/net/core/netmem_priv.h
@@ -5,19 +5,21 @@
  
  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);
+	struct netmem_desc *desc = netmem_to_nmdesc(netmem);
  
-	__netmem_clear_lsb(netmem)->pp_magic = 0;
+	WARN_ON_ONCE(desc->pp_magic & PP_DMA_INDEX_MASK);
+
+	desc->pp_magic = 0;
  }
  
  static inline bool netmem_is_pp(netmem_ref netmem)
@@ -27,13 +29,13 @@ 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 +45,7 @@ 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;
  }
@@ -51,12 +53,12 @@ static inline unsigned long netmem_get_dma_index(netmem_ref netmem)
  static inline void netmem_set_dma_index(netmem_ref netmem,
  					unsigned long id)
  {
-	unsigned long magic;
+	struct netmem_desc *desc;
  
  	if (WARN_ON_ONCE(netmem_is_net_iov(netmem)))
  		return;
  
-	magic = netmem_get_pp_magic(netmem) | (id << PP_DMA_INDEX_SHIFT);
-	__netmem_clear_lsb(netmem)->pp_magic = magic;
+	desc = netmem_to_nmdesc(netmem);
+	desc->pp_magic |= id << PP_DMA_INDEX_SHIFT;
  }
  #endif