Re: [PATCH RFC 2/8] fs: add a global device to super block hash table

[Christian Brauner]

On Tue, Jun 16, 2026 at 02:34:43PM +0200, Christoph Hellwig wrote:
> 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.

It already did.

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

We need a simple device number to superblock mapping anyway and that can
simply be centralized in the vfs. And it can work with anon device
numbers and block device numbers uniformly.