[PATCH RFC v2 1/3] mm: Add TAIL_MAPPING_REUSED_MAX

[Peter Xu <peterx@redhat.com>]

Tail pages have a sanity check on ->mapping fields, not all of them but
only upon index>2.  It's because we reused ->mapping fields of the tail
pages index=0,1 for other things.

Define a macro for "max index of tail pages that got ->mapping field
reused" on top of folio definition, because when we grow folio tail pages
we'd want to boost this too together.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 include/linux/mm_types.h | 9 +++++++++
 mm/huge_memory.c         | 6 +++---
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 291c05cacd48..3f2b0d46f5d6 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -248,6 +248,15 @@ static inline struct page *encoded_page_ptr(struct encoded_page *page)
 	return (struct page *)(~ENCODE_PAGE_BITS & (unsigned long)page);
 }
 
+/*
+ * This macro defines the maximum tail pages (of a folio) that can have the
+ * page->mapping field reused (offset 12 for 32bits, or 24 for 64bits).
+ *
+ * When the tail page's mapping field reused, it'll be exempted from
+ * ->mapping poisoning and checks.  Also see the macro TAIL_MAPPING.
+ */
+#define  TAIL_MAPPING_REUSED_MAX  (2)
+
 /**
  * struct folio - Represents a contiguous set of bytes.
  * @flags: Identical to the page flags.
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 0b709d2c46c6..72f244e16dcb 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2444,9 +2444,9 @@ static void __split_huge_page_tail(struct page *head, int tail,
 			 (1L << PG_dirty) |
 			 LRU_GEN_MASK | LRU_REFS_MASK));
 
-	/* ->mapping in first and second tail page is replaced by other uses */
-	VM_BUG_ON_PAGE(tail > 2 && page_tail->mapping != TAIL_MAPPING,
-			page_tail);
+	/* ->mapping in <=TAIL_MAPPING_REUSED_MAX tail pages are reused */
+	VM_BUG_ON_PAGE(tail > TAIL_MAPPING_REUSED_MAX &&
+		       page_tail->mapping != TAIL_MAPPING, page_tail);
 	page_tail->mapping = head->mapping;
 	page_tail->index = head->index + tail;
 
-- 
2.41.0