[PATCH 01/49] mm/sparse: fix vmemmap accounting imbalance on memory hotplug error

[Muchun Song <songmuchun@bytedance.com>]

In section_activate(), if populate_section_memmap() fails, the error
handling path calls section_deactivate() to roll back the state. This
approach introduces an accounting imbalance.

Since the commit c3576889d87b ("mm: fix accounting of memmap pages"),
memmap pages are accounted for only after populate_section_memmap()
succeeds. However, section_deactivate() unconditionally decrements the
vmemmap account. Consequently, a failure in populate_section_memmap()
leads to a negative offset (underflow) in the system's vmemmap tracking.

We can fix this by ensuring that the vmemmap accounting is incremented
immediately before checking for the success of populate_section_memmap().
If populate_section_memmap() fails, the subsequent call to
section_deactivate() will decrement the accounting, perfectly offsetting
the increment and maintaining balance.

Fixes: c3576889d87b ("mm: fix accounting of memmap pages")
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 mm/sparse-vmemmap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index 6eadb9d116e4..ee27d0c0efe2 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c
@@ -822,11 +822,11 @@ static struct page * __meminit section_activate(int nid, unsigned long pfn,
 		return pfn_to_page(pfn);
 
 	memmap = populate_section_memmap(pfn, nr_pages, nid, altmap, pgmap);
+	memmap_pages_add(DIV_ROUND_UP(nr_pages * sizeof(struct page), PAGE_SIZE));
 	if (!memmap) {
 		section_deactivate(pfn, nr_pages, altmap);
 		return ERR_PTR(-ENOMEM);
 	}
-	memmap_pages_add(DIV_ROUND_UP(nr_pages * sizeof(struct page), PAGE_SIZE));
 
 	return memmap;
 }
-- 
2.20.1