Re: [PATCH net-next v9 2/8] netmem: introduce utility APIs to use struct netmem_desc

On 7/10/25 19:11, Mina Almasry wrote:

On Thu, Jul 10, 2025 at 1:28 AM Byungchul Park [off-list ref] wrote:

quoted

To eliminate the use of the page pool fields in struct page, the page
pool code should use netmem descriptor and APIs instead.

However, some code e.g. __netmem_to_page() is still used to access the
page pool fields e.g. ->pp via struct page, which should be changed so
as to access them via netmem descriptor, struct netmem_desc instead,
since the fields no longer will be available in struct page.

Introduce utility APIs to make them easy to use struct netmem_desc as
descriptor.  The APIs are:

    1. __netmem_to_nmdesc(), to convert netmem_ref to struct netmem_desc,
       but unsafely without checking if it's net_iov or system memory.

    2. netmem_to_nmdesc(), to convert netmem_ref to struct netmem_desc,
       safely with checking if it's net_iov or system memory.

    3. nmdesc_to_page(), to convert struct netmem_desc to struct page,
       assuming struct netmem_desc overlays on struct page.

    4. page_to_nmdesc(), to convert struct page to struct netmem_desc,
       assuming struct netmem_desc overlays on struct page, allowing only
       head page to be converted.

    5. nmdesc_adress(), to get its virtual address corresponding to the
       struct netmem_desc.

Signed-off-by: Byungchul Park <byungchul@sk.com>
---
  include/net/netmem.h | 41 +++++++++++++++++++++++++++++++++++++++++
  1 file changed, 41 insertions(+)

diff --git a/include/net/netmem.h b/include/net/netmem.h
index 535cf17b9134..ad9444be229a 100644
--- a/include/net/netmem.h
+++ b/include/net/netmem.h

@@ -198,6 +198,32 @@ static inline struct page *netmem_to_page(netmem_ref netmem)
         return __netmem_to_page(netmem);
  }

+/**
+ * __netmem_to_nmdesc - unsafely get pointer to the &netmem_desc backing
+ * @netmem
+ * @netmem: netmem reference to convert
+ *
+ * Unsafe version of netmem_to_nmdesc(). When @netmem is always backed
+ * by system memory, performs faster and generates smaller object code
+ * (no check for the LSB, no WARN). When @netmem points to IOV, provokes
+ * undefined behaviour.
+ *
+ * Return: pointer to the &netmem_desc (garbage if @netmem is not backed
+ * by system memory).
+ */
+static inline struct netmem_desc *__netmem_to_nmdesc(netmem_ref netmem)
+{
+       return (__force struct netmem_desc *)netmem;
+}
+

Does a netmem_desc represent the pp fields shared between struct page
and struct net_iov, or does netmem_desc represent paged kernel memory?
If the former, I don't think we need a safe and unsafe version of this
helper, since netmem_ref always has netmem_desc fields underneath. If
the latter, then this helper should not exist at all. We should not
allow casting netmem_ref to a netmem_desc without first checking if
it's a net_iov.

+1, and...

To be honest the cover letter should come up with a detailed
explanation of (a) what are the current types (b) what are the new
types (c) what are the relationships between the types, so these
questions stop coming up.

quoted

+static inline struct netmem_desc *netmem_to_nmdesc(netmem_ref netmem)
+{
+       if (WARN_ON_ONCE(netmem_is_net_iov(netmem)))
+               return NULL;

... specifically this function should work with net_iov.

-- 
Pavel Begunkov