[PATCH net-next v6 1/5] mm: add a signature in struct page

[Matteo Croce <mcroce@xxxxxxxxxxxxxxxxxxx>]

From: Matteo Croce <mcroce@xxxxxxxxxxxxx>

This is needed by the page_pool to avoid recycling a page not allocated
via page_pool.

The page->signature field is aliased to page->lru.next and
page->compound_head, but it can't be set by mistake because the
signature value is a bad pointer, and can't trigger a false positive
in PageTail() because the last bit is 0.

Co-developed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Signed-off-by: Matteo Croce <mcroce@xxxxxxxxxxxxx>
---
 include/linux/mm.h       | 12 +++++++-----
 include/linux/mm_types.h | 12 +++++++++++-
 include/linux/poison.h   |  3 +++
 net/core/page_pool.c     |  6 ++++++
 4 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 322ec61d0da7..4ecfd8472a17 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1668,10 +1668,12 @@ struct address_space *page_mapping(struct page *page);
 static inline bool page_is_pfmemalloc(const struct page *page)
 {
 	/*
-	 * Page index cannot be this large so this must be
-	 * a pfmemalloc page.
+	 * This is not a tail page; compound_head of a head page is unused
+	 * at return from the page allocator, and will be overwritten
+	 * by callers who do not care whether the page came from the
+	 * reserves.
 	 */
-	return page->index == -1UL;
+	return page->compound_head & BIT(1);
 }
 
 /*
@@ -1680,12 +1682,12 @@ static inline bool page_is_pfmemalloc(const struct page *page)
  */
 static inline void set_page_pfmemalloc(struct page *page)
 {
-	page->index = -1UL;
+	page->compound_head = BIT(1);
 }
 
 static inline void clear_page_pfmemalloc(struct page *page)
 {
-	page->index = 0;
+	page->compound_head = 0;
 }
 
 /*
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 5aacc1c10a45..09f90598ff63 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -96,6 +96,13 @@ struct page {
 			unsigned long private;
 		};
 		struct {	/* page_pool used by netstack */
+			/**
+			 * @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;
 			/**
 			 * @dma_addr: might require a 64-bit value on
 			 * 32-bit architectures.
@@ -130,7 +137,10 @@ struct page {
 			};
 		};
 		struct {	/* Tail pages of compound page */
-			unsigned long compound_head;	/* Bit zero is set */
+			/* Bit zero is set
+			 * Bit one if pfmemalloc page
+			 */
+			unsigned long compound_head;
 
 			/* First tail page only */
 			unsigned char compound_dtor;
diff --git a/include/linux/poison.h b/include/linux/poison.h
index aff1c9250c82..d62ef5a6b4e9 100644
--- a/include/linux/poison.h
+++ b/include/linux/poison.h
@@ -78,4 +78,7 @@
 /********** security/ **********/
 #define KEY_DESTROY		0xbd
 
+/********** net/core/page_pool.c **********/
+#define PP_SIGNATURE		(0x40 + POISON_POINTER_DELTA)
+
 #endif
diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index 3c4c4c7a0402..e1321bc9d316 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -17,6 +17,7 @@
 #include <linux/dma-mapping.h>
 #include <linux/page-flags.h>
 #include <linux/mm.h> /* for __put_page() */
+#include <linux/poison.h>
 
 #include <trace/events/page_pool.h>
 
@@ -221,6 +222,8 @@ static struct page *__page_pool_alloc_page_order(struct page_pool *pool,
 		return NULL;
 	}
 
+	page->pp_magic |= PP_SIGNATURE;
+
 	/* Track how many pages are held 'in-flight' */
 	pool->pages_state_hold_cnt++;
 	trace_page_pool_state_hold(pool, page, pool->pages_state_hold_cnt);
@@ -263,6 +266,7 @@ static struct page *__page_pool_alloc_pages_slow(struct page_pool *pool,
 			put_page(page);
 			continue;
 		}
+		page->pp_magic |= PP_SIGNATURE;
 		pool->alloc.cache[pool->alloc.count++] = page;
 		/* Track how many pages are held 'in-flight' */
 		pool->pages_state_hold_cnt++;
@@ -341,6 +345,8 @@ void page_pool_release_page(struct page_pool *pool, struct page *page)
 			     DMA_ATTR_SKIP_CPU_SYNC);
 	page_pool_set_dma_addr(page, 0);
 skip_dma_unmap:
+	page->pp_magic = 0;
+
 	/* This may be the last page returned, releasing the pool, so
 	 * it is not safe to reference pool afterwards.
 	 */
-- 
2.31.1