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Date: Tue, 16 Jun 2026 14:34:43 +0200
From: Christoph Hellwig <hch@lst.de>
To: Christian Brauner <brauner@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>, Jan Kara <jack@suse.cz>,
	Jens Axboe <axboe@kernel.dk>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, Carlos Maiolino <cem@kernel.org>,
	linux-xfs@vger.kernel.org, Chris Mason <clm@fb.com>,
	David Sterba <dsterba@suse.com>, linux-btrfs@vger.kernel.org,
	Theodore Ts'o <tytso@mit.edu>, linux-ext4@vger.kernel.org,
	Gao Xiang <xiang@kernel.org>, linux-erofs@lists.ozlabs.org
Subject: Re: [PATCH RFC 2/8] fs: add a global device to super block hash
 table
Message-ID: <20260616123443.GA21024@lst.de>
References: <20260602-work-super-bdev_holder_global-v1-0-bb0fd82f3861@kernel.org> <20260602-work-super-bdev_holder_global-v1-2-bb0fd82f3861@kernel.org>
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On Tue, Jun 02, 2026 at 12:10:08PM +0200, Christian Brauner wrote:
> fs_holder_ops recovers the owning superblock from bdev->bd_holder, which
> forces the holder to be exactly one superblock and prevents several
> superblocks from sharing one block device. That's what erofs is doing.
> 
> Introduce a global dev_t-keyed rhltable mapping each block device to the
> superblock(s) using it. The holder argument becomes purely the block
> layer's exclusivity token (a superblock, or a file_system_type for
> shared devices) and is no longer needed by the fs specific callbacks.

Err, no.  block devices need to have a specific owner.  If erofs wants
to share a device between superblock it needs to come up with an entity
that owns the block devices which is not a superblock.

IMHO sharing devices between superblocks is a bad idea, but that ship
has sailed, but please keep it contained inside of erofs.