Re: [RFC PATCH net-next v5 07/14] page_pool: devmem support

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On 12/18/23 02:40, Mina Almasry wrote:
Convert netmem to be a union of struct page and struct netmem. Overload
the LSB of struct netmem* to indicate that it's a net_iov, otherwise
it's a page.

Currently these entries in struct page are rented by the page_pool and
used exclusively by the net stack:

struct {
	unsigned long pp_magic;
	struct page_pool *pp;
	unsigned long _pp_mapping_pad;
	unsigned long dma_addr;
	atomic_long_t pp_ref_count;
};

Mirror these (and only these) entries into struct net_iov and implement
netmem helpers that can access these common fields regardless of
whether the underlying type is page or net_iov.
Implement checks for net_iov in netmem helpers which delegate to mm
APIs, to ensure net_iov are never passed to the mm stack.

Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx>

---

RFCv5:
- Use netmem instead of page* with LSB set.
- Use pp_ref_count for refcounting net_iov.
- Removed many of the custom checks for netmem.

v1:
- Disable fragmentation support for iov properly.
- fix napi_pp_put_page() path (Yunsheng).
- Use pp_frag_count for devmem refcounting.

---
  include/net/netmem.h            | 145 ++++++++++++++++++++++++++++++--
  include/net/page_pool/helpers.h |  25 +++---
  net/core/page_pool.c            |  26 +++---
  net/core/skbuff.c               |   9 +-
  4 files changed, 164 insertions(+), 41 deletions(-)

diff --git a/include/net/netmem.h b/include/net/netmem.h
index 31f338f19da0..7557aecc0f78 100644
--- a/include/net/netmem.h
+++ b/include/net/netmem.h
@@ -12,11 +12,47 @@
/* net_iov */ +DECLARE_STATIC_KEY_FALSE(page_pool_mem_providers);
+
+/*  We overload the LSB of the struct page pointer to indicate whether it's
+ *  a page or net_iov.
+ */
+#define NET_IOV 0x01UL
+
  struct net_iov {
+	unsigned long __unused_padding;
+	unsigned long pp_magic;
+	struct page_pool *pp;
  	struct dmabuf_genpool_chunk_owner *owner;
  	unsigned long dma_addr;
+	atomic_long_t pp_ref_count;
  };

I wonder if it would be better to extract a common sub-struct
used in struct page, struct_group_tagged can help to avoid
touching old code:

struct page {
	unsigned long flags;
	union {
		...
		struct_group_tagged(<struct_name>, ...,
			/**
			 * @pp_magic: magic value to avoid recycling non
			 * page_pool allocated pages.
			 */
			unsigned long pp_magic;
			struct page_pool *pp;
			unsigned long _pp_mapping_pad;
			unsigned long dma_addr;
			atomic_long_t pp_ref_count;
		);
	};
}

struct net_iov {
	unsigned long pad;
	struct <struct_name> p;
};


A bit of a churn with the padding and nesting net_iov but looks
sturdier. No duplication, and you can just check positions of the
structure instead of per-field NET_IOV_ASSERT_OFFSET, which you
have to not forget to update e.g. when adding a new field. Also,
with the change __netmem_clear_lsb can return a pointer to that
structure, casting struct net_iov when it's a page is a bit iffy.

And the next question would be whether it'd be a good idea to encode
iov vs page not by setting a bit but via one of the fields in the
structure, maybe pp_magic.

With that said I'm a bit concerned about the net_iov size. If each
represents 4096 bytes and you're registering 10MB, then you need
30 pages worth of memory just for the iov array. Makes kvmalloc
a must even for relatively small sizes.

And the final bit, I don't believe the overlay is necessary in
this series. Optimisations are great, but this one is a bit more on
the controversial side. Unless I missed something and it does make
things easier, it might make sense to do it separately later.


+/* These fields in struct page are used by the page_pool and net stack:
+ *
+ *	struct {
+ *		unsigned long pp_magic;
+ *		struct page_pool *pp;
+ *		unsigned long _pp_mapping_pad;
+ *		unsigned long dma_addr;
+ *		atomic_long_t pp_ref_count;
+ *	};
+ *
+ * We mirror the page_pool fields here so the page_pool can access these fields
+ * without worrying whether the underlying fields belong to a page or net_iov.
+ *
+ * The non-net stack fields of struct page are private to the mm stack and must
+ * never be mirrored to net_iov.
+ */
+#define NET_IOV_ASSERT_OFFSET(pg, iov)             \
+	static_assert(offsetof(struct page, pg) == \
+		      offsetof(struct net_iov, iov))
+NET_IOV_ASSERT_OFFSET(pp_magic, pp_magic);
+NET_IOV_ASSERT_OFFSET(pp, pp);
+NET_IOV_ASSERT_OFFSET(dma_addr, dma_addr);
+NET_IOV_ASSERT_OFFSET(pp_ref_count, pp_ref_count);
+#undef NET_IOV_ASSERT_OFFSET
+
  static inline struct dmabuf_genpool_chunk_owner *
  net_iov_owner(const struct net_iov *niov)
  {
@@ -47,19 +83,25 @@ net_iov_binding(const struct net_iov *niov)
  struct netmem {
  	union {
  		struct page page;
-
-		/* Stub to prevent compiler implicitly converting from page*
-		 * to netmem_t* and vice versa.
-		 *
-		 * Other memory type(s) net stack would like to support
-		 * can be added to this union.
-		 */
-		void *addr;
+		struct net_iov niov;
  	};
  };
...

--
Pavel Begunkov



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