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

On 7/14/25 05:23, Byungchul Park wrote:

quoted

On Sat, Jul 12, 2025 at 03:39:59PM +0100, Pavel Begunkov wrote:

quoted

On 7/10/25 09:28, Byungchul Park wrote:

quoted

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