Split the utility function prep_compound_page() into head and tail
counterparts, and use them accordingly.

This is in preparation for sharing the storage for compound page
metadata.

Signed-off-by: Joao Martins <joao.m.mart...@oracle.com>
Acked-by: Mike Kravetz <mike.krav...@oracle.com>
Reviewed-by: Dan Williams <dan.j.willi...@intel.com>
Reviewed-by: Muchun Song <songmuc...@bytedance.com>
---
 mm/page_alloc.c | 30 ++++++++++++++++++++----------
 1 file changed, 20 insertions(+), 10 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c5952749ad40..20b9db0cf97c 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -726,23 +726,33 @@ void free_compound_page(struct page *page)
        free_the_page(page, compound_order(page));
 }
 
+static void prep_compound_head(struct page *page, unsigned int order)
+{
+       set_compound_page_dtor(page, COMPOUND_PAGE_DTOR);
+       set_compound_order(page, order);
+       atomic_set(compound_mapcount_ptr(page), -1);
+       if (hpage_pincount_available(page))
+               atomic_set(compound_pincount_ptr(page), 0);
+}
+
+static void prep_compound_tail(struct page *head, int tail_idx)
+{
+       struct page *p = head + tail_idx;
+
+       p->mapping = TAIL_MAPPING;
+       set_compound_head(p, head);
+}
+
 void prep_compound_page(struct page *page, unsigned int order)
 {
        int i;
        int nr_pages = 1 << order;
 
        __SetPageHead(page);
-       for (i = 1; i < nr_pages; i++) {
-               struct page *p = page + i;
-               p->mapping = TAIL_MAPPING;
-               set_compound_head(p, page);
-       }
+       for (i = 1; i < nr_pages; i++)
+               prep_compound_tail(page, i);
 
-       set_compound_page_dtor(page, COMPOUND_PAGE_DTOR);
-       set_compound_order(page, order);
-       atomic_set(compound_mapcount_ptr(page), -1);
-       if (hpage_pincount_available(page))
-               atomic_set(compound_pincount_ptr(page), 0);
+       prep_compound_head(page, order);
 }
 
 #ifdef CONFIG_DEBUG_PAGEALLOC
-- 
2.17.2


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