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Tue, 21 Oct 2025 12:04:50 -0700 (PDT) Received: from KASONG-MC4.tencent.com ([101.32.222.185]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-29246ebca5csm117664615ad.19.2025.10.21.12.04.45 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Tue, 21 Oct 2025 12:04:49 -0700 (PDT) From: Kairui Song To: linux-mm@kvack.org Cc: Andrew Morton , Baolin Wang , Hugh Dickins , Dev Jain , David Hildenbrand , Barry Song , Liam Howlett , Lorenzo Stoakes , Mariano Pache , Matthew Wilcox , Ryan Roberts , Zi Yan , linux-kernel@vger.kernel.org, Kairui Song , stable@vger.kernel.org Subject: [PATCH] mm/shmem: fix THP allocation size check and fallback Date: Wed, 22 Oct 2025 03:04:36 +0800 Message-ID: <20251021190436.81682-1-ryncsn@gmail.com> X-Mailer: git-send-email 2.51.0 Reply-To: Kairui Song 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" From: Kairui Song There are some problems with the code implementations of THP fallback. suitable_orders could be zero, and calling highest_order on a zero value returns an overflowed size. And the order check loop is updating the index value on every loop which may cause the index to be aligned by a larger value while the loop shrinks the order. And it forgot to try order 0 after the final loop. This is usually fine because shmem_add_to_page_cache ensures the shmem mapping is still sane, but it might cause many potential issues like allocating random folios into the random position in the map or return -ENOMEM by accident. This triggered some strange userspace errors [1], and shouldn't have happened in the first place. Cc: stable@vger.kernel.org Link: https://lore.kernel.org/linux-mm/CAMgjq7DqgAmj25nDUwwu1U2cSGSn8n4-Hqp= gottedy0S6YYeUw@mail.gmail.com/ [1] Fixes: e7a2ab7b3bb5d ("mm: shmem: add mTHP support for anonymous shmem") Signed-off-by: Kairui Song --- mm/shmem.c | 26 +++++++++++++++----------- 1 file changed, 15 insertions(+), 11 deletions(-) diff --git a/mm/shmem.c b/mm/shmem.c index b50ce7dbc84a..25303711f123 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -1824,6 +1824,9 @@ static unsigned long shmem_suitable_orders(struct ino= de *inode, struct vm_fault unsigned long pages; int order; =20 + if (!orders) + return 0; + if (vma) { orders =3D thp_vma_suitable_orders(vma, vmf->address, orders); if (!orders) @@ -1888,27 +1891,28 @@ static struct folio *shmem_alloc_and_add_folio(stru= ct vm_fault *vmf, if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) orders =3D 0; =20 - if (orders > 0) { - suitable_orders =3D shmem_suitable_orders(inode, vmf, - mapping, index, orders); + suitable_orders =3D shmem_suitable_orders(inode, vmf, + mapping, index, orders); =20 + if (suitable_orders) { order =3D highest_order(suitable_orders); - while (suitable_orders) { + do { pages =3D 1UL << order; - index =3D round_down(index, pages); - folio =3D shmem_alloc_folio(gfp, order, info, index); - if (folio) + folio =3D shmem_alloc_folio(gfp, order, info, round_down(index, pages)); + if (folio) { + index =3D round_down(index, pages); goto allocated; + } =20 if (pages =3D=3D HPAGE_PMD_NR) count_vm_event(THP_FILE_FALLBACK); count_mthp_stat(order, MTHP_STAT_SHMEM_FALLBACK); order =3D next_order(&suitable_orders, order); - } - } else { - pages =3D 1; - folio =3D shmem_alloc_folio(gfp, 0, info, index); + } while (suitable_orders); } + + pages =3D 1; + folio =3D shmem_alloc_folio(gfp, 0, info, index); if (!folio) return ERR_PTR(-ENOMEM); =20 --=20 2.51.0