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charset=UTF-8 Date: Tue, 22 Sep 2026 11:29:17 -0400 Message-Id: Subject: Re: [PATCH v4 1/4] mm/truncate: align truncation boundaries to mapping minimum folio order Cc: , , , , , , , , , , , , , , , , , , , , , , , To: "Zhang Yi" , From: "Zi Yan" X-Mailer: aerc 0.22.0 References: <20260922110703.468389-1-yi.zhang@huaweicloud.com> <20260922110703.468389-2-yi.zhang@huaweicloud.com> In-Reply-To: <20260922110703.468389-2-yi.zhang@huaweicloud.com> X-ClientProxiedBy: BN1PR12CA0024.namprd12.prod.outlook.com (2603:10b6:408:e1::29) To IA0PR12MB8374.namprd12.prod.outlook.com (2603:10b6:208:40e::7) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: IA0PR12MB8374:EE_|PH7PR12MB6935:EE_ X-MS-Office365-Filtering-Correlation-Id: bc1e9e98-2931-423b-ea03-08df18be454d X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|23010399003|7416014|376014|1800799024|56012099006|6133799003|10067099003|11063799006|4143699003|18002099003|22082099003; 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The original > boundaries in truncate_inode_pages_range() were based on page > granularity, so either boundary could land inside the min_order chunk at > its edge, and the truncation loop would drop that whole chunk, valid > out-of-range tail included. > > For example, a 64K (order-4) folio with min_order =3D 2 (16K) punched fro= m > offset 0 to 36K: > > split @p0 -> [p0-p3, p4-p7, p8-p15] # non-uniform, min_order > folio2 =3D p8-p15 # straddles: p8 in range, p9-p15 tail valid > 2nd split of folio2 -> [p8-p11, p12-p15] # success > end(old) =3D p9 # BUG: p9 inside [p8-p11] > loop truncates ... p8-p11 # p9-p11's valid tail is lost > > It has gone unnoticed so far for two reasons. A non-zero min_order is > only used by filesystems with a block or sector size larger than the > page size, and those either always write back the affected range before > punching a hole or truncating, or they carry filesystem private data on > dirty folios (e.g. buffer_head), which makes filemap_release_folio() > fail and folio_split() abort with -EBUSY, so the folio is never split > and the old start/end boundaries remain valid. The bug only becomes > reachable on paths that truncate dirty large folios without prior > writeback and without filesystem private data, such as the upcoming ext4 > iomap buffered I/O path. > > Align both start (rounded up) and end (rounded down) to the mapping > minimum folio order so they always fall on a folio boundary. > > Reported-by: Joanne Koong > Link: https://lore.kernel.org/linux-mm/CAJnrk1bQYUe6+1ryyJur5EEnZYrC+_5AY= sy=3DOWzVRgD4202y1g@mail.gmail.com/ > Fixes: e220917fa5077 ("mm: split a folio in minimum folio order chunks") > Suggested-by: Zi Yan > Signed-off-by: Zhang Yi > --- > mm/truncate.c | 18 ++++++++++++------ > 1 file changed, 12 insertions(+), 6 deletions(-) LGTM. Just a nit below. Reviewed-by: Zi Yan > > diff --git a/mm/truncate.c b/mm/truncate.c > index b58ba940be47..f9625bb4916f 100644 > --- a/mm/truncate.c > +++ b/mm/truncate.c > @@ -345,9 +345,11 @@ long mapping_evict_folio(struct address_space *mappi= ng, struct folio *folio) > * @lstart: offset from which to truncate > * @lend: offset to which to truncate (inclusive) > * > - * Truncate the page cache, removing the pages that are between > - * specified offsets (and zeroing out partial pages > - * if lstart or lend + 1 is not page aligned). > + * Truncate the page cache, removing the folios that are between specifi= ed > + * offsets (and zeroing out partial folios if lstart or lend + 1 is not > + * folio aligned). For mappings with a non-zero minimum folio order, th= e > + * boundaries are aligned inwards to 1 << min_order so the edge sub-foli= o > + * straddling the range is kept. > * > * Truncate takes two passes - the first pass is nonblocking. It will n= ot > * block on page locks and it will not block on writeback. The second p= ass > @@ -374,14 +376,14 @@ void truncate_inode_pages_range(struct address_spac= e *mapping, > int i; > struct folio *folio; > bool same_folio; > + pgoff_t min_nrpages =3D mapping_min_folio_nrpages(mapping); > =20 It is better to put it at the top (reverse christmas tree). --=20 Best Regards, Yan, Zi