Re: [PATCH] mm, page_pool: introduce a new page type for page pool in page type

On Tue, Jul 22, 2025 at 03:17:15PM -0700, Mina Almasry wrote:

On Sun, Jul 20, 2025 at 10:49 PM Byungchul Park [off-list ref] wrote:

quoted

Hi,

I focused on converting the existing APIs accessing ->pp_magic field to
page type APIs.  However, yes.  Additional works would better be
considered on top like:

   1. Adjust how to store and retrieve dma index.  Maybe network guys
      can work better on top.

   2. Move the sanity check for page pool in mm/page_alloc.c to on free.

   Byungchul

---8<---
From 7d207a1b3e9f4ff2a72f5b54b09e3ed0c4aaaca3 Mon Sep 17 00:00:00 2001
From: Byungchul Park <byungchul@sk.com>
Date: Mon, 21 Jul 2025 14:05:20 +0900
Subject: [PATCH] mm, page_pool: introduce a new page type for page pool in page type

->pp_magic field in struct page is current used to identify if a page
belongs to a page pool.  However, page type e.i. PGTY_netpp can be used
for that purpose.

Use the page type APIs e.g. PageNetpp(), __SetPageNetpp(), and
__ClearPageNetpp() instead, and remove the existing APIs accessing
->pp_magic e.g. page_pool_page_is_pp(), netmem_or_pp_magic(), and
netmem_clear_pp_magic() since they are totally replaced.

This work was inspired by the following link by Pavel:

[1] https://lore.kernel.org/all/582f41c0-2742-4400-9c81-0d46bf4e8314@gmail.com/ (local)

Suggested-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Byungchul Park <byungchul@sk.com>
---
 .../net/ethernet/mellanox/mlx5/core/en/xdp.c  |  2 +-
 include/linux/mm.h                            | 28 ++-----------------
 include/linux/page-flags.h                    |  6 ++++
 include/net/netmem.h                          |  2 +-
 mm/page_alloc.c                               |  4 +--
 net/core/netmem_priv.h                        | 16 ++---------
 net/core/page_pool.c                          | 10 +++++--
 7 files changed, 24 insertions(+), 44 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c b/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
index 5d51600935a6..def274f5c1ca 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c

@@ -707,7 +707,7 @@ static void mlx5e_free_xdpsq_desc(struct mlx5e_xdpsq *sq,
                                xdpi = mlx5e_xdpi_fifo_pop(xdpi_fifo);
                                page = xdpi.page.page;

-                               /* No need to check page_pool_page_is_pp() as we
+                               /* No need to check PageNetpp() as we
                                 * know this is a page_pool page.
                                 */
                                page_pool_recycle_direct(pp_page_to_nmdesc(page)->pp,

diff --git a/include/linux/mm.h b/include/linux/mm.h
index ae50c1641bed..736061749535 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h

@@ -4135,10 +4135,9 @@ int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status);
  * DMA mapping IDs for page_pool
  *
  * When DMA-mapping a page, page_pool allocates an ID (from an xarray) and
- * stashes it in the upper bits of page->pp_magic. We always want to be able to
- * unambiguously identify page pool pages (using page_pool_page_is_pp()). Non-PP
- * pages can have arbitrary kernel pointers stored in the same field as pp_magic
- * (since it overlaps with page->lru.next), so we must ensure that we cannot
+ * stashes it in the upper bits of page->pp_magic. Non-PP pages can have
+ * arbitrary kernel pointers stored in the same field as pp_magic (since
+ * it overlaps with page->lru.next), so we must ensure that we cannot
  * mistake a valid kernel pointer with any of the values we write into this
  * field.
  *

@@ -4168,25 +4167,4 @@ int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status);

 #define PP_DMA_INDEX_MASK GENMASK(PP_DMA_INDEX_BITS + PP_DMA_INDEX_SHIFT - 1, \
                                  PP_DMA_INDEX_SHIFT)
-
-/* Mask used for checking in page_pool_page_is_pp() below. page->pp_magic is
- * OR'ed with PP_SIGNATURE after the allocation in order to preserve bit 0 for
- * the head page of compound page and bit 1 for pfmemalloc page, as well as the
- * bits used for the DMA index. page_is_pfmemalloc() is checked in
- * __page_pool_put_page() to avoid recycling the pfmemalloc page.
- */
-#define PP_MAGIC_MASK ~(PP_DMA_INDEX_MASK | 0x3UL)
-
-#ifdef CONFIG_PAGE_POOL
-static inline bool page_pool_page_is_pp(const struct page *page)
-{
-       return (page->pp_magic & PP_MAGIC_MASK) == PP_SIGNATURE;
-}
-#else
-static inline bool page_pool_page_is_pp(const struct page *page)
-{
-       return false;
-}
-#endif
-
 #endif /* _LINUX_MM_H */

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 4fe5ee67535b..906ba7c9e372 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h

@@ -957,6 +957,7 @@ enum pagetype {
        PGTY_zsmalloc           = 0xf6,
        PGTY_unaccepted         = 0xf7,
        PGTY_large_kmalloc      = 0xf8,
+       PGTY_netpp              = 0xf9,

        PGTY_mapcount_underflow = 0xff
 };

@@ -1101,6 +1102,11 @@ PAGE_TYPE_OPS(Zsmalloc, zsmalloc, zsmalloc)
 PAGE_TYPE_OPS(Unaccepted, unaccepted, unaccepted)
 FOLIO_TYPE_OPS(large_kmalloc, large_kmalloc)

+/*
+ * Marks page_pool allocated pages.
+ */
+PAGE_TYPE_OPS(Netpp, netpp, netpp)
+
 /**
  * PageHuge - Determine if the page belongs to hugetlbfs
  * @page: The page to test.

diff --git a/include/net/netmem.h b/include/net/netmem.h
index f7dacc9e75fd..3667334e16e7 100644
--- a/include/net/netmem.h
+++ b/include/net/netmem.h

@@ -298,7 +298,7 @@ static inline struct net_iov *__netmem_clear_lsb(netmem_ref netmem)
  */
 #define pp_page_to_nmdesc(p)                                           \
 ({                                                                     \
-       DEBUG_NET_WARN_ON_ONCE(!page_pool_page_is_pp(p));               \
+       DEBUG_NET_WARN_ON_ONCE(!PageNetpp(p));                          \
        __pp_page_to_nmdesc(p);                                         \
 })

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2ef3c07266b3..71c7666e48a9 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c

@@ -898,7 +898,7 @@ static inline bool page_expected_state(struct page *page,
 #ifdef CONFIG_MEMCG
                        page->memcg_data |
 #endif
-                       page_pool_page_is_pp(page) |
+                       PageNetpp(page) |
                        (page->flags & check_flags)))
                return false;

@@ -925,7 +925,7 @@ static const char *page_bad_reason(struct page *page, unsigned long flags)
        if (unlikely(page->memcg_data))
                bad_reason = "page still charged to cgroup";
 #endif
-       if (unlikely(page_pool_page_is_pp(page)))
+       if (unlikely(PageNetpp(page)))
                bad_reason = "page_pool leak";
        return bad_reason;
 }

diff --git a/net/core/netmem_priv.h b/net/core/netmem_priv.h
index cd95394399b4..39a97703d9ed 100644
--- a/net/core/netmem_priv.h
+++ b/net/core/netmem_priv.h

@@ -8,21 +8,11 @@ static inline unsigned long netmem_get_pp_magic(netmem_ref netmem)
        return __netmem_clear_lsb(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;
-}
-
-static inline void netmem_clear_pp_magic(netmem_ref netmem)
-{
-       WARN_ON_ONCE(__netmem_clear_lsb(netmem)->pp_magic & PP_DMA_INDEX_MASK);
-
-       __netmem_clear_lsb(netmem)->pp_magic = 0;
-}
-
 static inline bool netmem_is_pp(netmem_ref netmem)
 {
-       return (netmem_get_pp_magic(netmem) & PP_MAGIC_MASK) == PP_SIGNATURE;
+       if (netmem_is_net_iov(netmem))
+               return true;

As Pavel alludes, this is dubious, and at least it's difficult to
reason about it.

There could be net_iovs that are not attached to pp, and should not be
treated as pp memory. These are in the devmem (and future net_iov) tx
paths.

We need a way to tell if a net_iov is pp or not. A couple of options:

1. We could have it such that if net_iov->pp is set, then the
netmem_is_pp == true, otherwise false.
2. We could implement a page-flags equivalent for net_iov.

Option #1 is simpler and is my preferred. To do that properly, you need to:

1. Make sure everywhere net_iovs are allocated that pp=NULL in the
non-pp case and pp=non NULL in the pp case. those callsites are
net_devmem_bind_dmabuf (devmem rx & tx path), io_zcrx_create_area
(io_uring rx path).

2. Change netmem_is_pp to check net_iov->pp in the net_iov case.

Good idea, but I'm not sure if I could work on it without consuming your
additional review efforts.  Can anyone add net_iov_is_pp() helper?

Or use the page type, Netpp, as an additional way to identify if it's a
pp page for system memory, keeping the current way using ->pp_magic.
So the page type, Netpp, is used for system memory, and ->pp_magic is
used for net_iov.  The clean up for ->pp_magic can be done if needed.

	Byungchul

--
Thanks,
Mina