Re: [PATCH net-next v9 3/8] page_pool: access ->pp_magic through struct netmem_desc in page_pool_page_is_pp()

[Pavel Begunkov <asml.silence@gmail.com>]

On 7/11/25 02:14, Byungchul Park wrote:
...>>> +#ifdef CONFIG_PAGE_POOL
>>> +/* XXX: This would better be moved to mm, once mm gets its way to
>>> + * identify the type of page for page pool.
>>> + */
>>> +static inline bool page_pool_page_is_pp(struct page *page)
>>> +{
>>> +       struct netmem_desc *desc = page_to_nmdesc(page);
>>> +
>>> +       return (desc->pp_magic & PP_MAGIC_MASK) == PP_SIGNATURE;
>>> +}
>>
>> pages can be pp pages (where they have pp fields inside of them) or
>> non-pp pages (where they don't have pp fields inside them, because
>> they were never allocated from the page_pool).
>>
>> Casting a page to a netmem_desc, and then checking if the page was a
>> pp page doesn't makes sense to me on a fundamental level. The
>> netmem_desc is only valid if the page was a pp page in the first
>> place. Maybe page_to_nmdesc should reject the cast if the page is not
>> a pp page or something.
> 
> Right, as you already know, the current mainline code already has the
> same problem but we've been using the werid way so far, in other words,
> mm code is checking if it's a pp page or not by using ->pp_magic, but
> it's ->lur, ->buddy_list, or ->pcp_list if it's not a pp page.
> 
> Both the mainline code and this patch can make sense *only if* it's
> actually a pp page.  It's unevitable until mm provides a way to identify
> the type of page for page pool.  Thoughts?
Question to mm folks, can we add a new PGTY for page pool and use
that to filter page pool originated pages? Like in the incomplete
and untested diff below?


commit 8fc2347fb3ff4a3fc7929c70a5a21e1128935d4a
Author: Pavel Begunkov <asml.silence@gmail.com>
Date:   Sat Jul 12 14:29:52 2025 +0100

     net/mm: use PGTY for tracking page pool pages
     
     Currently, we use page->pp_magic to determine whether a page belongs to
     a page pool. It's not ideal as the field is aliased with other page
     types, and thus needs to to rely on elaborated rules to work. Add a new
     page type for page pool.

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 0ef2ba0c667a..975a013f1f17 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -4175,7 +4175,7 @@ int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status);
  #ifdef CONFIG_PAGE_POOL
  static inline bool page_pool_page_is_pp(struct page *page)
  {
-	return (page->pp_magic & PP_MAGIC_MASK) == PP_SIGNATURE;
+	return PageNetpp(page);
  }
  #else
  static inline bool page_pool_page_is_pp(struct page *page)
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 4fe5ee67535b..9bd1dfded2fc 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 de1d95f04076..20f5dbb08149 100644
--- a/include/net/netmem.h
+++ b/include/net/netmem.h
@@ -113,6 +113,8 @@ static inline bool netmem_is_net_iov(const netmem_ref netmem)
   */
  static inline struct page *__netmem_to_page(netmem_ref netmem)
  {
+	DEBUG_NET_WARN_ON_ONCE(netmem_is_net_iov(netmem));
+
  	return (__force struct page *)netmem;
  }
  
diff --git a/net/core/netmem_priv.h b/net/core/netmem_priv.h
index cd95394399b4..e38c64da1a78 100644
--- a/net/core/netmem_priv.h
+++ b/net/core/netmem_priv.h
@@ -13,16 +13,11 @@ 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;
+	return page_pool_page_is_pp(netmem_to_page(netmem));
  }
  
  static inline void netmem_set_pp(netmem_ref netmem, struct page_pool *pool)
diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index 05e2e22a8f7c..52120e2912a6 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -371,6 +371,13 @@ struct page_pool *page_pool_create(const struct page_pool_params *params)
  }
  EXPORT_SYMBOL(page_pool_create);
  
+static void page_pool_set_page_pp_info(struct page_pool *pool,
+				       struct page *page)
+{
+	__SetPageNetpp(page);
+	page_pool_set_pp_info(page_to_netmem(page));
+}
+
  static void page_pool_return_netmem(struct page_pool *pool, netmem_ref netmem);
  
  static noinline netmem_ref page_pool_refill_alloc_cache(struct page_pool *pool)
@@ -534,7 +541,7 @@ static struct page *__page_pool_alloc_page_order(struct page_pool *pool,
  	}
  
  	alloc_stat_inc(pool, slow_high_order);
-	page_pool_set_pp_info(pool, page_to_netmem(page));
+	page_pool_set_page_pp_info(pool, page);
  
  	/* Track how many pages are held 'in-flight' */
  	pool->pages_state_hold_cnt++;
@@ -579,7 +586,7 @@ static noinline netmem_ref __page_pool_alloc_netmems_slow(struct page_pool *pool
  			continue;
  		}
  
-		page_pool_set_pp_info(pool, netmem);
+		page_pool_set_page_pp_info(pool, __netmem_to_page(netmem));
  		pool->alloc.cache[pool->alloc.count++] = netmem;
  		/* Track how many pages are held 'in-flight' */
  		pool->pages_state_hold_cnt++;
@@ -654,7 +661,6 @@ 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);
  
  	/* Ensuring all pages have been split into one fragment initially:
  	 * page_pool_set_pp_info() is only called once for every page when it
@@ -669,7 +675,6 @@ 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);
  	netmem_set_pp(netmem, NULL);
  }
  
@@ -730,8 +735,11 @@ static void page_pool_return_netmem(struct page_pool *pool, netmem_ref netmem)
  	trace_page_pool_state_release(pool, netmem, count);
  
  	if (put) {
+		struct page *page = netmem_to_page(netmem);
+
  		page_pool_clear_pp_info(netmem);
-		put_page(netmem_to_page(netmem));
+		__ClearPageNetpp(page);
+		put_page(page);
  	}
  	/* An optimization would be to call __free_pages(page, pool->p.order)
  	 * knowing page is not part of page-cache (thus avoiding a

-- 
Pavel Begunkov