linux-ext4 - Re: [RFC[RAP]] fuse: use fs-iomap for better performance so we can containerize ext4

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Message-ID: <CAOQ4uxg-HT9ZA4UdQsD40z4THp9wBJw=MPHBSnWUCbOA+Mc0hA@mail.gmail.com>
Date: Wed, 11 Jun 2025 10:54:16 +0200
From: Amir Goldstein <amir73il@...il.com>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: linux-fsdevel <linux-fsdevel@...r.kernel.org>, John@...ves.net, bernd@...ernd.com, 
	miklos@...redi.hu, joannelkoong@...il.com, Josef Bacik <josef@...icpanda.com>, 
	linux-ext4 <linux-ext4@...r.kernel.org>, "Theodore Ts'o" <tytso@....edu>
Subject: Re: [RFC[RAP]] fuse: use fs-iomap for better performance so we can
 containerize ext4

> > > Does the nodeid for the root directory have to be FUSE_ROOT_ID?
> >
> > Yeh, I think that's the case, otherwise FUSE_INIT would need to
> > tell the kernel the root nodeid, because there is no lookup to
> > return the root nodeid.
> >
> > > I guess
> > > for ext4 that's not a big deal since ext2 inode #1 is the badblocks file
> > > which cannot be accessed from userspace anyway.
> > >
> >
> > As long as inode #1 is reserved it should be fine.
> > just need to refine the rules of the one-to-one mapping with
> > this exception.
>
> Or just make it so that passthrough_ino filesystems can specify the
> rootdir inumber?
>

There is already a mount option 'rootmode' for st_mode of root inode
so I suppose we could add the rootino mount option.

Note that currently fuse_fill_super_common() instantiates the root inode
before negotiating FUSE_INIT with the server.

Thanks,
Amir.