Grepping for "prep_compound_page" leaves on clueless how devdax gets its
compound pages initialized.

Let's add a comment that might help finding this open-coded
prep_compound_page() initialization more easily.

Further, let's be less smart about the ordering of initialization and just
perform the prep_compound_head() call after all tail pages were
initialized: just like prep_compound_page() does.

No need for a lengthy comment then: again, just like prep_compound_page().

Note that prep_compound_head() already does initialize stuff in page[2]
through prep_compound_head() that successive tail page initialization
will overwrite: _deferred_list, and on 32bit _entire_mapcount and
_pincount. Very likely 32bit does not apply, and likely nobody ever ends
up testing whether the _deferred_list is empty.

So it shouldn't be a fix at this point, but certainly something to clean
up.

Signed-off-by: David Hildenbrand <redacted>
---
 mm/mm_init.c | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/mm/mm_init.c b/mm/mm_init.c
index 5c21b3af216b2..708466c5b2cc9 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c

@@ -1091,6 +1091,10 @@ static void __ref memmap_init_compound(struct page *head,
 	unsigned long pfn, end_pfn = head_pfn + nr_pages;
 	unsigned int order = pgmap->vmemmap_shift;
 
+	/*
+	 * This is an open-coded prep_compound_page() whereby we avoid
+	 * walking pages twice by initializing them in the same go.
+	 */
 	__SetPageHead(head);
 	for (pfn = head_pfn + 1; pfn < end_pfn; pfn++) {
 		struct page *page = pfn_to_page(pfn);

@@ -1098,15 +1102,8 @@ static void __ref memmap_init_compound(struct page *head,
 		__init_zone_device_page(page, pfn, zone_idx, nid, pgmap);
 		prep_compound_tail(head, pfn - head_pfn);
 		set_page_count(page, 0);
-
-		/*
-		 * The first tail page stores important compound page info.
-		 * Call prep_compound_head() after the first tail page has
-		 * been initialized, to not have the data overwritten.
-		 */
-		if (pfn == head_pfn + 1)
-			prep_compound_head(head, order);
 	}
+	prep_compound_head(head, order);
 }
 
 void __ref memmap_init_zone_device(struct zone *zone,

-- 
2.50.1