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. 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 --- mm/hugetlb.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/mm/hugetlb.c b/mm/hugetlb.c index 327eaa4074d3..5be36a888e70 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -2915,6 +2915,7 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma, map_chg_state map_chg; int ret, idx; struct hugetlb_cgroup *h_cg = NULL; + struct hugetlb_cgroup *h_cg_rsvd = NULL; gfp_t gfp = htlb_alloc_mask(h) | __GFP_RETRY_MAYFAIL; idx = hstate_index(h); @@ -2965,7 +2966,7 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma, */ if (map_chg) { 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; } @@ -3007,7 +3008,7 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma, */ if (map_chg) { hugetlb_cgroup_commit_charge_rsvd(idx, pages_per_huge_page(h), - h_cg, folio); + h_cg_rsvd, folio); } spin_unlock_irq(&hugetlb_lock); @@ -3059,7 +3060,7 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma, out_uncharge_cgroup_reservation: if (map_chg) hugetlb_cgroup_uncharge_cgroup_rsvd(idx, pages_per_huge_page(h), - h_cg); + h_cg_rsvd); out_subpool_put: /* * put page to subpool iff the quota of subpool's rsv_hpages is used -- 2.43.0