mm: move page->deferred_list to folio->_deferred_list

Remove the entire block of definitions for the second tail page, and add
the deferred list to the struct folio.  This actually moves _deferred_list
to a different offset in struct folio because I don't see a need to
include the padding.

This lets us use list_for_each_entry_safe() in deferred_split_scan()
and avoid a number of calls to compound_head().

Link: https://lkml.kernel.org/r/20230111142915.1001531-25-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 4 insertions, 5 deletions

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index a1341fdcf666..aacfcb02606f 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -295,11 +295,10 @@ static inline bool thp_migration_supported(void)
 
 static inline struct list_head *page_deferred_list(struct page *page)
 {
-	/*
-	 * See organization of tail pages of compound page in
-	 * "struct page" definition.
-	 */
-	return &page[2].deferred_list;
+	struct folio *folio = (struct folio *)page;
+
+	VM_BUG_ON_FOLIO(folio_order(folio) < 2, folio);
+	return &folio->_deferred_list;
 }
 
 #else /* CONFIG_TRANSPARENT_HUGEPAGE */