Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <158230810644.2185128.16726948836367716086.stgit@warthog.procyon.org.uk>
 <1582316494.3376.45.camel@HansenPartnership.com> <CAOssrKehjnTwbc6A1VagM5hG_32hy3mXZenx_PdGgcUGxYOaLQ@mail.gmail.com>
 <1582556135.3384.4.camel@HansenPartnership.com>
In-Reply-To: <1582556135.3384.4.camel@HansenPartnership.com>
From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Mon, 24 Feb 2020 16:28:40 +0100
Message-ID: <CAJfpegsk6BsVhUgHNwJgZrqcNP66wS0fhCXo_2sLt__goYGPWg@mail.gmail.com>
Subject: Re: [PATCH 00/17] VFS: Filesystem information and notifications [ver #17]
To:     James Bottomley <James.Bottomley@hansenpartnership.com>
Cc:     Miklos Szeredi <mszeredi@redhat.com>,
        David Howells <dhowells@redhat.com>,
        viro <viro@zeniv.linux.org.uk>, Ian Kent <raven@themaw.net>,
        Christian Brauner <christian@brauner.io>,
        Jann Horn <jannh@google.com>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, Feb 24, 2020 at 3:55 PM James Bottomley
<James.Bottomley@hansenpartnership.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