[RFC PATCH] mm: hugetlb: Fix incorrect fallback for subpool

[Wupeng Ma]

During our testing with hugetlb subpool enabled, we observe that
hstate->resv_huge_pages may underflow into negative values. Root cause
analysis reveals a race condition in subpool reservation fallback handling
as follow:

hugetlb_reserve_pages()
    /* Attempt subpool reservation */
    gbl_reserve = hugepage_subpool_get_pages(spool, chg);

    /* Global reservation may fail after subpool allocation */
    if (hugetlb_acct_memory(h, gbl_reserve) < 0)
        goto out_put_pages;

out_put_pages:
    /* This incorrectly restores reservation to subpool */
    hugepage_subpool_put_pages(spool, chg);

When hugetlb_acct_memory() fails after subpool allocation, the current
implementation over-commits subpool reservations by returning the full
'chg' value instead of the actual allocated 'gbl_reserve' amount. This
discrepancy propagates to global reservations during subsequent releases,
eventually causing resv_huge_pages underflow.

This problem can be trigger easily with the following steps:
1. reverse hugepage for hugeltb allocation
2. mount hugetlbfs with min_size to enable hugetlb subpool
3. alloc hugepages with two task(make sure the second will fail due to
   insufficient amount of hugepages)
4. with for a few seconds and repeat step 3 which will make
   hstate->resv_huge_pages to go below zero.

To fix this problem, return corrent amount of pages to subpool during the
fallback after hugepage_subpool_get_pages is called.

Fixes: 1c5ecae3a93f ("hugetlbfs: add minimum size accounting to subpools")
Signed-off-by: Wupeng Ma <mawupeng1@xxxxxxxxxx>
---
 mm/hugetlb.c | 28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 318624c96584..dc4449592d00 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2987,7 +2987,7 @@ 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;
+	long retval, gbl_chg, gbl_reserve;
 	map_chg_state map_chg;
 	int ret, idx;
 	struct hugetlb_cgroup *h_cg = NULL;
@@ -3140,8 +3140,16 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
 		hugetlb_cgroup_uncharge_cgroup_rsvd(idx, pages_per_huge_page(h),
 						    h_cg);
 out_subpool_put:
-	if (map_chg)
-		hugepage_subpool_put_pages(spool, 1);
+	/*
+	 * 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);
+	}
+
+
 out_end_reservation:
 	if (map_chg != MAP_CHG_ENFORCED)
 		vma_end_reservation(h, vma, addr);
@@ -6949,7 +6957,7 @@ bool hugetlb_reserve_pages(struct inode *inode,
 					struct vm_area_struct *vma,
 					vm_flags_t vm_flags)
 {
-	long chg = -1, add = -1;
+	long chg = -1, add = -1, spool_resv, gbl_resv;
 	struct hstate *h = hstate_inode(inode);
 	struct hugepage_subpool *spool = subpool_inode(inode);
 	struct resv_map *resv_map;
@@ -7084,8 +7092,16 @@ bool hugetlb_reserve_pages(struct inode *inode,
 	return true;
 
 out_put_pages:
-	/* put back original number of pages, chg */
-	(void)hugepage_subpool_put_pages(spool, chg);
+	spool_resv = chg - gbl_reserve;
+	if (spool_resv) {
+		/* put sub pool's reservation back, chg - gbl_reserve */
+		gbl_resv = hugepage_subpool_put_pages(spool, spool_resv);
+		/*
+		 * subpool's reserved pages can not be put back due to race,
+		 * return to hstate.
+		 */
+		hugetlb_acct_memory(h, -gbl_resv);
+	}
 out_uncharge_cgroup:
 	hugetlb_cgroup_uncharge_cgroup_rsvd(hstate_index(h),
 					    chg * pages_per_huge_page(h), h_cg);
-- 
2.43.0