[PATCH] mm/hugetlb: fix surplus pages in dissolve_free_huge_page()

[Jinjiang Tu <tujinjiang@xxxxxxxxxx>]

In dissolve_free_huge_page(), free huge pages are dissolved without
adjusting surplus count. However, free huge pages may be accounted as
surplus pages, and will lead to wrong surplus count.

Steps to reproduce:
1) create 64 huge pages for node1
2) run a program to reserve (don't consume) all the huge pages
3) echo 0 > nr_huge_pages for node1. After this step, free huge pages in
Node1 are surplus.
4) create 80 huge pages for node0
5) offline memory of node1 and kill the program in step 2)

The result:
           Node0     Node1
total       80        0
free        80        0
surplus     0         61

To fix it, adjust surplus when destroying huge pages if the node has
surplus pages in dissolve_free_hugetlb_folio().

The result with this patch:
           Node0     Node1
total       80        0
free        80        0
surplus     0         0

Fixes: c8721bbbdd36 ("mm: memory-hotplug: enable memory hotplug to handle hugepage")
Signed-off-by: Jinjiang Tu <tujinjiang@xxxxxxxxxx>
---
 mm/hugetlb.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 163190e89ea1..2a24ade9d157 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2135,6 +2135,8 @@ int dissolve_free_hugetlb_folio(struct folio *folio)
 
 	if (!folio_ref_count(folio)) {
 		struct hstate *h = folio_hstate(folio);
+		bool adjust_surplus = false;
+
 		if (!available_huge_pages(h))
 			goto out;
 
@@ -2157,7 +2159,9 @@ int dissolve_free_hugetlb_folio(struct folio *folio)
 			goto retry;
 		}
 
-		remove_hugetlb_folio(h, folio, false);
+		if (h->surplus_huge_pages_node[folio_nid(folio)])
+			adjust_surplus = true;
+		remove_hugetlb_folio(h, folio, adjust_surplus);
 		h->max_huge_pages--;
 		spin_unlock_irq(&hugetlb_lock);
 
@@ -2177,7 +2181,7 @@ int dissolve_free_hugetlb_folio(struct folio *folio)
 			rc = hugetlb_vmemmap_restore_folio(h, folio);
 			if (rc) {
 				spin_lock_irq(&hugetlb_lock);
-				add_hugetlb_folio(h, folio, false);
+				add_hugetlb_folio(h, folio, adjust_surplus);
 				h->max_huge_pages++;
 				goto out;
 			}
-- 
2.43.0