linux-kernel - Re: [PATCH 00/17] VFS: Filesystem information and notifications [ver #17]

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Date:   Mon, 24 Feb 2020 16:28:40 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     James Bottomley <James.Bottomley@...senpartnership.com>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        David Howells <dhowells@...hat.com>,
        viro <viro@...iv.linux.org.uk>, Ian Kent <raven@...maw.net>,
        Christian Brauner <christian@...uner.io>,
        Jann Horn <jannh@...gle.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 00/17] VFS: Filesystem information and notifications [ver #17]

On Mon, Feb 24, 2020 at 3:55 PM James Bottomley
<James.Bottomley@...senpartnership.com> wrote:

> Once it's table driven, certainly a sysfs directory becomes possible.
> The problem with ST_DEV is filesystems like btrfs and xfs that may have
> multiple devices.

For XFS there's always  a single sb->s_dev though, that's what st_dev
will be set to on all files.

Btrfs subvolume is sort of a lightweight superblock, so basically all
such st_dev's are aliases of the same master superblock.  So lookup of
all subvolume st_dev's could result in referencing the same underlying
struct super_block (just like /proc/$PID will reference the same
underlying task group regardless of which of the task group member's
PID is used).

Having this info in sysfs would spare us a number of issues that a set
of new syscalls would bring.  The question is, would that be enough,
or is there a reason that sysfs can't be used to present the various
filesystem related information that fsinfo is supposed to present?

Thanks,
Miklos