Re: [PATCH 4/3] Prepare transhuge pages properly

[Matthew Wilcox <willy@infradead.org>]

Bill pointed out I'd forgotten to call prep_transhuge_page().  I'll
fold this into some of the other commits, but this is what I'm thinking
of doing in case anyone has a better idea:

Basically, I prefer being able to do this:

-	return alloc_pages(gfp, order);
+	return prep_transhuge_page(alloc_pages(gfp, order));

to this:

+	struct page *page;
-	return alloc_pages(gfp, order);
+	page = alloc_pages(gfp, order);
+	if (page && (gfp & __GFP_COMP))
+		prep_transhuge_page(page);
+	return page;

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 45ede62aa85b..159e63438806 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -153,7 +153,7 @@ extern unsigned long thp_get_unmapped_area(struct file *filp,
 		unsigned long addr, unsigned long len, unsigned long pgoff,
 		unsigned long flags);
 
-extern void prep_transhuge_page(struct page *page);
+extern struct page *prep_transhuge_page(struct page *page);
 extern void free_transhuge_page(struct page *page);
 
 bool can_split_huge_page(struct page *page, int *pextra_pins);
@@ -294,7 +294,10 @@ static inline bool transhuge_vma_suitable(struct vm_area_struct *vma,
 	return false;
 }
 
-static inline void prep_transhuge_page(struct page *page) {}
+static inline struct page *prep_transhuge_page(struct page *page)
+{
+	return page;
+}
 
 #define transparent_hugepage_flags 0UL
 
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 72101811524c..8b9d672d868c 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -215,7 +215,7 @@ struct page *__page_cache_alloc_order(gfp_t gfp, unsigned int order)
 {
 	if (order > 0)
 		gfp |= __GFP_COMP;
-	return alloc_pages(gfp, order);
+	return prep_transhuge_page(alloc_pages(gfp, order));
 }
 #endif
 
diff --git a/mm/filemap.c b/mm/filemap.c
index a7fa3a50f750..c2b11799b968 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -986,11 +986,12 @@ struct page *__page_cache_alloc_order(gfp_t gfp, unsigned int order)
 			cpuset_mems_cookie = read_mems_allowed_begin();
 			n = cpuset_mem_spread_node();
 			page = __alloc_pages_node(n, gfp, order);
+			prep_transhuge_page(page);
 		} while (!page && read_mems_allowed_retry(cpuset_mems_cookie));
 
 		return page;
 	}
-	return alloc_pages(gfp, order);
+	return prep_transhuge_page(alloc_pages(gfp, order));
 }
 EXPORT_SYMBOL(__page_cache_alloc_order);
 #endif
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 483b07b2d6ae..3961af907dd7 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -502,15 +502,20 @@ static inline struct list_head *page_deferred_list(struct page *page)
 	return &page[2].deferred_list;
 }
 
-void prep_transhuge_page(struct page *page)
+struct page *prep_transhuge_page(struct page *page)
 {
+	if (!page || compound_order(page) == 0)
+		return page;
 	/*
-	 * we use page->mapping and page->indexlru in second tail page
+	 * we use page->mapping and page->index in second tail page
 	 * as list_head: assuming THP order >= 2
 	 */
+	BUG_ON(compound_order(page) == 1);
 
 	INIT_LIST_HEAD(page_deferred_list(page));
 	set_compound_page_dtor(page, TRANSHUGE_PAGE_DTOR);
+
+	return page;
 }
 
 static unsigned long __thp_get_unmapped_area(struct file *filp, unsigned long len,