mm/mm_init: make memmap_init_compound() look more like prep_compound_page()

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 comment to describe the initialization order: again, just
like prep_compound_page().

Link: https://lkml.kernel.org/r/20250901150359.867252-10-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/mm_init.c b/mm/mm_init.c
index 5c21b3af216b212a2f4642f1a503ae4206e15e5e..df614556741a44659b58ac247ad054bbc26bdbfd 100644 (file)
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -1091,6 +1091,12 @@ static void __ref memmap_init_compound(struct page *head,
        unsigned long pfn, end_pfn = head_pfn + nr_pages;
        unsigned int order = pgmap->vmemmap_shift;
 
+       /*
+        * We have to initialize the pages, including setting up page links.
+        * prep_compound_page() does not take care of that, so instead we
+        * open-code prep_compound_page() so we can take care of initializing
+        * the pages in the same go.
+        */
        __SetPageHead(head);
        for (pfn = head_pfn + 1; pfn < end_pfn; pfn++) {
                struct page *page = pfn_to_page(pfn);
@@ -1098,15 +1104,8 @@ 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,