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linux-fsdevel@vger.kernel.org, Eero Kelly , Andrew Battat , Adam Bratschi-Kaye Subject: Re: [External Sender] [REGRESSION] madvise(MADV_REMOVE) corrupts pages in THP-backed MAP_SHARED memfd (bisected to 7460b470a131) Date: Fri, 27 Feb 2026 14:29:25 -0500 X-Mailer: MailMate (2.0r6290) Message-ID: <2EECA9B4-E1F0-42FF-9E61-3E4AC4B4DC13@nvidia.com> In-Reply-To: References: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: BY5PR03CA0030.namprd03.prod.outlook.com (2603:10b6:a03:1e0::40) To DS7PR12MB9473.namprd12.prod.outlook.com (2603:10b6:8:252::5) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS7PR12MB9473:EE_|MN0PR12MB6080:EE_ X-MS-Office365-Filtering-Correlation-Id: ca251c79-0168-4647-5731-08de763686ff X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|366016; X-Microsoft-Antispam-Message-Info: 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 26 Feb 2026, at 16:16, Bas van Dijk wrote: > On Thu, Feb 26, 2026 at 10:06=E2=80=AFPM Zi Yan wrote: >> >> On 26 Feb 2026, at 15:49, Zi Yan wrote: >> >>> On 26 Feb 2026, at 15:34, Bas van Dijk wrote: >>> >>>> #regzbot introduced: 7460b470a131f985a70302a322617121efdd7caa >>>> >>>> Hey folks, >>>> >>>> We discovered madvise(MADV_REMOVE) on a 4KiB range within a >>>> huge-page-backed MAP_SHARED memfd region corrupts nearby pages. >>>> >>>> Using the reproducible test in >>>> https://github.com/dfinity/thp-madv-remove-test this was bisected to t= he >>>> first bad commit: >>>> >>>> commit 7460b470a131f985a70302a322617121efdd7caa >>>> Author: Zi Yan >>>> Date: Fri Mar 7 12:40:00 2025 -0500 >>>> >>>> mm/truncate: use folio_split() in truncate operation >>>> >>>> v7.0-rc1 still has the regression. >>>> >>>> The repo mentioned above explains how to reproduce the regression and >>>> contains the necessary logs of failed runs on 7460b470a131 and v7.0-rc= 1, as >>>> well as a successful run on its parent 4b94c18d1519. >>> >>> Thanks for the report. I will look into it. >> >> Can you also share your kernel config file? I just ran the reproducer an= d >> could not trigger the corruption. > > Sure, I just ran `nix build > .#linux_6_14_first_bad_7460b470a131.configfile -o kernel.config` which > produced: > > https://github.com/dfinity/thp-madv-remove-test/blob/master/kernel.config Hi Bas, Can you try the patch below? It fixes the issue locally. I was able to use your app to reproduce the issue after change my shmem THP config from never to always. Thanks. >From 03b75f017ffe6cf556fefbd44f44655bf4a9af48 Mon Sep 17 00:00:00 2001 From: Zi Yan Date: Fri, 27 Feb 2026 14:11:36 -0500 Subject: [PATCH] mm/huge_memory: fix folio_split() race condition with folio_try_get() During a pagecache folio split, the values in the related xarray should not be changed from the original folio at xarray split time until all after-split folios are ready and stored in the xarray. Otherwise, a parallel folio_try_get() can see stale values in the xarray and a stale value can be a unfrozen after-split folio. This leads to a wrong folio returned to userspace. Signed-off-by: Zi Yan --- mm/huge_memory.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index d4ca8cfd7f9d..3d5bf3bb8a3e 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -3628,6 +3628,7 @@ static int __split_unmapped_folio(struct folio *folio= , int new_order, const bool is_anon =3D folio_test_anon(folio); int old_order =3D folio_order(folio); int start_order =3D split_type =3D=3D SPLIT_TYPE_UNIFORM ? new_order : ol= d_order - 1; + struct folio *origin_folio =3D folio; int split_order; /* @@ -3653,7 +3654,13 @@ static int __split_unmapped_folio(struct folio *foli= o, int new_order, xas_split(xas, folio, old_order); else { xas_set_order(xas, folio->index, split_order); - xas_try_split(xas, folio, old_order); + /* + * use the original folio, so that a parallel + * folio_try_get() waits on it until xarray is + * updated with after-split folios and + * the original one is unfreezed + */ + xas_try_split(xas, origin_folio, old_order); if (xas_error(xas)) return xas_error(xas); } --=20 2.51.0 Best Regards, Yan, Zi