[RFC mm v5 1/2] page_pool: check nmdesc->pp to see its usage as page pool for net_iov not page-backed

[Byungchul Park]

Currently, the condition 'page->pp_magic == PP_SIGNATURE' is used to
determine if a page belongs to a page pool.  However, with the planned
removal of ->pp_magic, we will instead leverage the page_type in struct
page, such as PGTY_netpp, for this purpose.

That works for page-backed network memory.  However, for net_iov not
page-backed, the identification cannot be based on the page_type.
Instead, nmdesc->pp can be used to see if it belongs to a page pool, by
making sure nmdesc->pp is NULL otherwise.

For net_iov not page-backed, initialize it using nmdesc->pp = NULL in
net_devmem_bind_dmabuf() and using kvmalloc_array(__GFP_ZERO) in
io_zcrx_create_area() so that netmem_is_pp() can check if nmdesc->pp is
!NULL to confirm its usage as page pool.

Signed-off-by: Byungchul Park <byungchul@xxxxxx>
Reviewed-by: Mina Almasry <almasrymina@xxxxxxxxxx>
---
 net/core/devmem.c      |  1 +
 net/core/netmem_priv.h |  8 ++++++++
 net/core/page_pool.c   | 16 ++++++++++++++--
 3 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/net/core/devmem.c b/net/core/devmem.c
index d9de31a6cc7f..f81b700f1fd1 100644
--- a/net/core/devmem.c
+++ b/net/core/devmem.c
@@ -291,6 +291,7 @@ net_devmem_bind_dmabuf(struct net_device *dev,
 			niov = &owner->area.niovs[i];
 			niov->type = NET_IOV_DMABUF;
 			niov->owner = &owner->area;
+			niov->desc.pp = NULL;
 			page_pool_set_dma_addr_netmem(net_iov_to_netmem(niov),
 						      net_devmem_get_dma_addr(niov));
 			if (direction == DMA_TO_DEVICE)
diff --git a/net/core/netmem_priv.h b/net/core/netmem_priv.h
index 23175cb2bd86..5561fd556bc5 100644
--- a/net/core/netmem_priv.h
+++ b/net/core/netmem_priv.h
@@ -22,6 +22,14 @@ static inline void netmem_clear_pp_magic(netmem_ref netmem)
 
 static inline bool netmem_is_pp(netmem_ref netmem)
 {
+	/* net_iov may be part of a page pool.  For net_iov, ->pp in
+	 * net_iov.desc can be used to determine if the pages belong to
+	 * a page pool.  Ensure that the ->pp either points to its page
+	 * pool or is set to NULL if it does not.
+	 */
+	if (netmem_is_net_iov(netmem))
+		return !!netmem_to_nmdesc(netmem)->pp;
+
 	return (netmem_get_pp_magic(netmem) & PP_MAGIC_MASK) == PP_SIGNATURE;
 }
 
diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index 1a5edec485f1..2756b78754b0 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -699,7 +699,13 @@ s32 page_pool_inflight(const struct page_pool *pool, bool strict)
 void page_pool_set_pp_info(struct page_pool *pool, netmem_ref netmem)
 {
 	netmem_set_pp(netmem, pool);
-	netmem_or_pp_magic(netmem, PP_SIGNATURE);
+
+	/* For page-backed, pp_magic is used to identify if it's pp.
+	 * For net_iov, it's ensured nmdesc->pp is non-NULL if it's pp
+	 * and nmdesc->pp is NULL if it's not.
+	 */
+	if (!netmem_is_net_iov(netmem))
+		netmem_or_pp_magic(netmem, PP_SIGNATURE);
 
 	/* Ensuring all pages have been split into one fragment initially:
 	 * page_pool_set_pp_info() is only called once for every page when it
@@ -714,7 +720,13 @@ void page_pool_set_pp_info(struct page_pool *pool, netmem_ref netmem)
 
 void page_pool_clear_pp_info(netmem_ref netmem)
 {
-	netmem_clear_pp_magic(netmem);
+	/* For page-backed, pp_magic is used to identify if it's pp.
+	 * For net_iov, it's ensured nmdesc->pp is non-NULL if it's pp
+	 * and nmdesc->pp is NULL if it's not.
+	 */
+	if (!netmem_is_net_iov(netmem))
+		netmem_clear_pp_magic(netmem);
+
 	netmem_set_pp(netmem, NULL);
 }
 
-- 
2.17.1