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Wong" CC: hch , "linux-xfs@vger.kernel.org" Subject: Re: [bug report] xfs/802 failure due to mssing fstype report by lsblk Thread-Topic: [bug report] xfs/802 failure due to mssing fstype report by lsblk Thread-Index: AQHcl0QyMGi0b3KRKkOnzTIETMr5/bV18CqAgAO+3ICAADcfAIAABeSAgAAYG4CAAS9wAIAASEUA Date: Tue, 10 Feb 2026 06:19:21 +0000 Message-ID: References: <20260206173805.GY7712@frogsfrogsfrogs> <20260209060716.GL1535390@frogsfrogsfrogs> <20260209062821.GA9021@lst.de> <20260210020040.GC7712@frogsfrogsfrogs> In-Reply-To: <20260210020040.GC7712@frogsfrogsfrogs> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: authentication-results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=wdc.com; x-ms-publictraffictype: Email x-ms-traffictypediagnostic: SN7PR04MB8532:EE_|IA3PR04MB9307:EE_ x-ms-office365-filtering-correlation-id: 66966904-fb5a-4c73-2847-08de686c545c x-ld-processed: b61c8803-16f3-4c35-9b17-6f65f441df86,ExtAddr wdcipoutbound: EOP-TRUE x-ms-exchange-senderadcheck: 1 x-ms-exchange-antispam-relay: 0 x-microsoft-antispam: BCL:0;ARA:13230040|19092799006|376014|366016|1800799024|38070700021; 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Wong wrote: > On Mon, Feb 09, 2026 at 07:54:38AM +0000, Shinichiro Kawasaki wrote: > > On Feb 09, 2026 / 07:28, hch wrote: > > > On Sun, Feb 08, 2026 at 10:07:16PM -0800, Darrick J. Wong wrote: > > > > Waitaminute, how can you even format xfs on nullblk to run fstests? > > > > Isn't that the bdev that silently discards everything written to it= , and > > > > returns zero on reads?? > > >=20 > > > nullblk can be used with or without a backing store. In the former > > > case it will not always return zeroes on reads obviously. > >=20 > > Yes, null_blk has the "memory_backed" parameter. When 1 is set to this,= data > > written to the null_blk device is kept and read back. I create two 8GiB= null_blk > > devices enabling this memory_backed option, and use them as TEST_DEV an= d > > SCRATCH_DEV for the regular xfs test runs. >=20 > Huh, ok. Just out of curiosity, does blkid (in cache mode) /ever/ see > the xfs filesystem? I'm wondering if there's a race, a slow utility, or > if whatever builds the blkid cache sees that it's nullblk and ignores > it? I tried the experement below, using /dev/nullb1 formatted as xfs: # Clear blkid cache $ sudo rm /run/blkid/blkid.tab # Call blkid, but normal user can not parse superblock, then can not get fs= type. $ blkid --match-tag=3DTYPE /dev/nullb1 # Call blkid with superuser privilege. It can get fstype, but does not cach= e it, # since --probe option is specified. $ sudo blkid --probe --match-tag=3DTYPE /dev/nullb1 /dev/nullb1: TYPE=3D"xfs" # Still normal user can not get fstype since fstype is not cached. $ blkid --match-tag=3DTYPE /dev/nullb1 # Call blkid as superuser without --probe option. It caches the fstype. $sudo blkid --match-tag=3DTYPE /dev/nullb1 /dev/nullb1: TYPE=3D"xfs" # Now normal user can get fstype referring to the cache $ blkid --match-tag=3DTYPE /dev/nullb1 /dev/nullb1: TYPE=3D"xfs" Based on this result, my understanding is that blkid caches its superblock parse results when --probe, or -p option, is not specified. As far as I git grep util-linux, this behavior does not change for null_blk. Anyway, I think blkid with --probe option is good for fstests usage, since = it directly checks the superblock of the target block devices.=