[PATCH 1/4] mm: hugetlb: Fix subpool usage leak on allocation failure

[Ackerley Tng via B4 Relay <devnull+ackerleytng.google.com@kernel.org>]

From: Ackerley Tng <ackerleytng@google.com>

When alloc_hugetlb_folio() fails early (e.g. buddy allocation failure or
hugetlb cgroup charging failure) and gbl_chg == 1 (meaning a reservation
was not used, but a global page was allocated instead), the subpool page
acquired via hugepage_subpool_get_pages() must still be returned.

Currently, the error path out_subpool_put: only calls
hugepage_subpool_put_pages() if !gbl_chg is true. If gbl_chg is 1, it
skips it, permanently leaking the subpool's used_hpages counter.

Fix this by calling hugepage_subpool_put_pages() unconditionally if
map_chg is true. Only call hugetlb_acct_memory() to adjust global
reservations if !gbl_chg is true, to prevent underflowing the global
resv_huge_pages counter.

Fixes: a833a693a490e ("mm: hugetlb: fix incorrect fallback for subpool")
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 mm/hugetlb.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 571212b80835e..e607dcbb2fc3e 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2858,7 +2858,7 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
 	struct hugepage_subpool *spool = subpool_vma(vma);
 	struct hstate *h = hstate_vma(vma);
 	struct folio *folio;
-	long retval, gbl_chg, gbl_reserve;
+	long retval, gbl_chg;
 	map_chg_state map_chg;
 	int ret, idx;
 	struct hugetlb_cgroup *h_cg = NULL;
@@ -3009,13 +3009,11 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
 		hugetlb_cgroup_uncharge_cgroup_rsvd(idx, pages_per_huge_page(h),
 						    h_cg_rsvd);
 out_subpool_put:
-	/*
-	 * put page to subpool iff the quota of subpool's rsv_hpages is used
-	 * during hugepage_subpool_get_pages.
-	 */
-	if (map_chg && !gbl_chg) {
-		gbl_reserve = hugepage_subpool_put_pages(spool, 1);
-		hugetlb_acct_memory(h, -gbl_reserve);
+	if (map_chg) {
+		long gbl_reserve = hugepage_subpool_put_pages(spool, 1);
+
+		if (!gbl_chg)
+			hugetlb_acct_memory(h, -gbl_reserve);
 	}
 
 

-- 
2.55.0.795.g602f6c329a-goog