6.6-stable review patch. If anyone has any objections, please let me know. ------------------ From: Deepanshu Kartikey [ Upstream commit 15807d0ddde37407af72859426b654f3d1972b00 ] In alloc_hugetlb_folio(), a single h_cg pointer is used for both the rsvd and non-rsvd hugetlb cgroup charges. When map_chg is set, hugetlb_cgroup_charge_cgroup_rsvd() stores the charged cgroup in h_cg, but the immediately following hugetlb_cgroup_charge_cgroup() overwrites h_cg with the non-rsvd cgroup pointer. As a result, hugetlb_cgroup_commit_charge_rsvd() stores the wrong (non-rsvd) cgroup pointer into the folio's rsvd slot. When the folio is later freed, free_huge_folio() unconditionally calls both hugetlb_cgroup_uncharge_folio() and hugetlb_cgroup_uncharge_folio_rsvd(). The rsvd uncharge reads back the wrong cgroup from the folio and decrements a counter that was never charged for that cgroup, causing a page_counter underflow: page_counter underflow: -512 nr_pages=512 WARNING: mm/page_counter.c:61 at page_counter_cancel Fix this by introducing a separate h_cg_rsvd pointer exclusively for the rsvd charge path, keeping the rsvd and non-rsvd charges fully independent through their charge, commit, and error uncharge paths. Link: https://lore.kernel.org/20260328065534.346053-1-kartikey406@gmail.com Fixes: 08cf9faf7558 ("hugetlb_cgroup: support noreserve mappings") Reported-by: syzbot+226c1f947186f8fef796@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=226c1f947186f8fef796 Signed-off-by: Deepanshu Kartikey Reviewed-by: Muchun Song Cc: David Hildenbrand Cc: Oscar Salvador Cc: Mina Almasry Cc: Signed-off-by: Andrew Morton Signed-off-by: Sasha Levin --- mm/hugetlb.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/mm/hugetlb.c b/mm/hugetlb.c index cca79f0e78f70..8bfaaa84bb3f7 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -3058,6 +3058,7 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma, long gbl_chg; int ret, idx; struct hugetlb_cgroup *h_cg = NULL; + struct hugetlb_cgroup *h_cg_rsvd = NULL; bool deferred_reserve; idx = hstate_index(h); @@ -3090,7 +3091,7 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma, deferred_reserve = map_chg || avoid_reserve; if (deferred_reserve) { ret = hugetlb_cgroup_charge_cgroup_rsvd( - idx, pages_per_huge_page(h), &h_cg); + idx, pages_per_huge_page(h), &h_cg_rsvd); if (ret) goto out_subpool_put; } @@ -3127,7 +3128,7 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma, */ if (deferred_reserve) { hugetlb_cgroup_commit_charge_rsvd(idx, pages_per_huge_page(h), - h_cg, folio); + h_cg_rsvd, folio); } spin_unlock_irq(&hugetlb_lock); @@ -3163,7 +3164,7 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma, out_uncharge_cgroup_reservation: if (deferred_reserve) hugetlb_cgroup_uncharge_cgroup_rsvd(idx, pages_per_huge_page(h), - h_cg); + h_cg_rsvd); out_subpool_put: if (map_chg || avoid_reserve) hugepage_subpool_put_pages(spool, 1); -- 2.53.0