[PATCH v4 5/5] mm/mm_init: Fix uninitialized struct pages for ZONE_DEVICE

[Muchun Song <songmuchun@bytedance.com>]

If DAX memory is hotplugged into an unoccupied subsection of an early
section, section_activate() reuses the unoptimized boot memmap.
However, compound_nr_pages() still assumes that vmemmap optimization is
in effect and initializes only the reduced number of struct pages. As a
result, the remaining tail struct pages are left uninitialized, which
can later lead to unexpected behavior or crashes.

Fix this by treating early sections as unoptimized when calculating how
many struct pages to initialize.

Fixes: 6fd3620b3428 ("mm/page_alloc: reuse tail struct pages for compound devmaps")
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 mm/mm_init.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/mm/mm_init.c b/mm/mm_init.c
index 9d0fe79a94de..3d5af40d0943 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -1056,10 +1056,17 @@ static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
  * of how the sparse_vmemmap internals handle compound pages in the lack
  * of an altmap. See vmemmap_populate_compound_pages().
  */
-static inline unsigned long compound_nr_pages(struct vmem_altmap *altmap,
+static inline unsigned long compound_nr_pages(unsigned long pfn,
+					      struct vmem_altmap *altmap,
 					      struct dev_pagemap *pgmap)
 {
-	if (!vmemmap_can_optimize(altmap, pgmap))
+	/*
+	 * If DAX memory is hot-plugged into an unoccupied subsection
+	 * of an early section, the unoptimized boot memmap is reused.
+	 * See section_activate().
+	 */
+	if (early_section(__pfn_to_section(pfn)) ||
+	    !vmemmap_can_optimize(altmap, pgmap))
 		return pgmap_vmemmap_nr(pgmap);
 
 	return VMEMMAP_RESERVE_NR * (PAGE_SIZE / sizeof(struct page));
@@ -1129,7 +1136,7 @@ void __ref memmap_init_zone_device(struct zone *zone,
 			continue;
 
 		memmap_init_compound(page, pfn, zone_idx, nid, pgmap,
-				     compound_nr_pages(altmap, pgmap));
+				     compound_nr_pages(pfn, altmap, pgmap));
 	}
 
 	/*
-- 
2.20.1