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([2405:201:682f:389d:24c5:800f:f8d7:ae7d]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-35c22db12f7sm6722723a91.13.2026.03.27.23.55.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 27 Mar 2026 23:55:43 -0700 (PDT) From: Deepanshu Kartikey To: muchun.song@linux.dev, osalvador@suse.de, david@kernel.org, akpm@linux-foundation.org Cc: mike.kravetz@oracle.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Deepanshu Kartikey , syzbot+226c1f947186f8fef796@syzkaller.appspotmail.com Subject: [PATCH] mm/hugetlb: fix hugetlb cgroup rsvd charge/uncharge mismatch Date: Sat, 28 Mar 2026 12:25:34 +0530 Message-ID: <20260328065534.346053-1-kartikey406@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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=3D512 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=3D226c1f947186f8fef796 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_stru= ct *vma, map_chg_state map_chg; int ret, idx; struct hugetlb_cgroup *h_cg =3D NULL; + struct hugetlb_cgroup *h_cg_rsvd =3D NULL; gfp_t gfp =3D htlb_alloc_mask(h) | __GFP_RETRY_MAYFAIL; =20 idx =3D hstate_index(h); @@ -2965,7 +2966,7 @@ struct folio *alloc_hugetlb_folio(struct vm_area_stru= ct *vma, */ if (map_chg) { ret =3D 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_stru= ct *vma, */ if (map_chg) { hugetlb_cgroup_commit_charge_rsvd(idx, pages_per_huge_page(h), - h_cg, folio); + h_cg_rsvd, folio); } =20 spin_unlock_irq(&hugetlb_lock); @@ -3059,7 +3060,7 @@ struct folio *alloc_hugetlb_folio(struct vm_area_stru= ct *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 --=20 2.43.0