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Date: Tue, 19 Dec 2023 13:19:11 +0100
From: Christoph Hellwig <hch@lst.de>
To: Gao Xiang <hsiangkao@linux.alibaba.com>
Cc: Christoph Hellwig <hch@lst.de>, Chris Mason <clm@fb.com>,
	Josef Bacik <josef@toxicpanda.com>, David Sterba <dsterba@suse.com>,
	"linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>,
	Christian Brauner <brauner@kernel.org>,
	Eric Biggers <ebiggers@kernel.org>,
	Johannes Thumshirn <Johannes.Thumshirn@wdc.com>
Subject: Re: [PATCH 5/5] btrfs: use the super_block as holder when mounting
 file systems
Message-ID: <20231219121911.GA21959@lst.de>
References: <20231218044933.706042-1-hch@lst.de> <20231218044933.706042-6-hch@lst.de> <04e599b9-5d6d-4ac0-bf74-da9bedfb585f@wdc.com> <20231218150234.GB19041@lst.de> <5dc7ecdd-8bd3-4016-b8da-49ff306bcd3c@linux.alibaba.com>
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On Tue, Dec 19, 2023 at 01:33:54PM +0800, Gao Xiang wrote:
>>> ext4, f2fs and xfs use the super_block, erofs uses 'sb->s_type' as well
>>> here. Reiser uses the journal and so does jfs. So while these two might
>>> not be the best examples in the world, all other is an exaggeration.
>>
>> As of 6.8-rc every file system but btrfs should be using the superblock.
>
> Just saw this by chance.  Currently EROFS uses 'sb->s_type' to refer
> external binary source devices (blobs) across different mounts.  Since
> these devices are treated readonly so such external sources can be
> shared between mounts as some shared data layer.

Makes sense for that somewhat unusual use case.  Note that this means
you can't really use blk_holder_ops based notifications from the block
driver, but for read-only devices that's not all that important anyway.