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From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: Steven Whitehouse <swhiteho@redhat.com>,
	Miklos Szeredi <miklos@szeredi.hu>
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>
Subject: Re: [PATCH 00/17] VFS: Filesystem information and notifications [ver #17]
Date: Tue, 25 Feb 2020 07:28:55 -0800	[thread overview]
Message-ID: <1582644535.3361.8.camel@HansenPartnership.com> (raw)
In-Reply-To: <a657a80e-8913-d1f3-0ffe-d582f5cb9aa2@redhat.com>

On Tue, 2020-02-25 at 12:13 +0000, Steven Whitehouse wrote:
> Hi,
> 
> On 24/02/2020 15:28, Miklos Szeredi wrote:
> > 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
> > 
> 
> We need a unique id for superblocks anyway. I had wondered about
> using s_dev some time back, but for the reasons mentioned earlier in
> this thread I think it might just land up being confusing and
> difficult to manage. While fake s_devs are created for sbs that don't
> have a device, I can't help thinking that something closer to
> ifindex, but for superblocks, is needed here. That would avoid the
> issue of which device number to use.
> 
> In fact we need that anyway for the notifications, since without
> that  there is a race that can lead to missing remounts of the same
> device, in  case a umount/mount pair is missed due to an overrun, and
> then fsinfo returns the same device as before, with potentially the
> same mount options too. So I think a unique id for a superblock is a
> generically useful feature, which would also allow for sensible sysfs
> directory naming, if required,

But would this be informative and useful for the user?  I'm sure we can
find a persistent id for a persistent superblock, but what about tmpfs
... that's going to have to change with every reboot.  It's going to be
remarkably inconvenient if I want to get fsinfo on /run to have to keep
finding what the id is.

The other thing a file descriptor does that sysfs doesn't is that it
solves the information leak: if I'm in a mount namespace that has no
access to certain mounts, I can't fspick them and thus I can't see the
information.  By default, with sysfs I can.

James


  reply	other threads:[~2020-02-25 15:29 UTC|newest]

Thread overview: 117+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-21 18:01 [PATCH 00/17] VFS: Filesystem information and notifications [ver #17] David Howells
2020-02-21 18:01 ` [PATCH 01/17] watch_queue: Add security hooks to rule on setting mount and sb watches " David Howells
2020-02-21 18:02 ` [PATCH 02/17] watch_queue: Implement mount topology and attribute change notifications " David Howells
2020-02-21 18:02 ` [PATCH 03/17] watch_queue: sample: Display mount tree " David Howells
2020-02-21 18:02 ` [PATCH 04/17] watch_queue: Introduce a non-repeating system-unique superblock ID " David Howells
2020-02-21 18:02 ` [PATCH 05/17] watch_queue: Add superblock notifications " David Howells
2020-02-21 18:02 ` [PATCH 06/17] watch_queue: sample: Display " David Howells
2020-02-21 18:02 ` [PATCH 07/17] fsinfo: Add fsinfo() syscall to query filesystem information " David Howells
2020-02-26  2:29   ` Aleksa Sarai
2020-02-28 14:44   ` David Howells
2020-02-21 18:02 ` [PATCH 08/17] fsinfo: Provide a bitmap of supported features " David Howells
2020-02-21 18:03 ` [PATCH 09/17] fsinfo: Allow fsinfo() to look up a mount object by ID " David Howells
2020-02-21 18:03 ` [PATCH 10/17] fsinfo: Allow mount information to be queried " David Howells
2020-03-04 14:58   ` Miklos Szeredi
2020-03-04 16:10   ` Miklos Szeredi
2020-02-21 18:03 ` [PATCH 11/17] fsinfo: sample: Mount listing program " David Howells
2020-02-21 18:03 ` [PATCH 12/17] fsinfo: Allow the mount topology propogation flags to be retrieved " David Howells
2020-02-21 18:03 ` [PATCH 13/17] fsinfo: Query superblock unique ID and notification counter " David Howells
2020-02-21 18:03 ` [PATCH 14/17] fsinfo: Add API documentation " David Howells
2020-02-21 18:03 ` [PATCH 15/17] fsinfo: Add support for AFS " David Howells
2020-02-21 18:03 ` [PATCH 16/17] fsinfo: Add example support for Ext4 " David Howells
2020-02-21 18:04 ` [PATCH 17/17] fsinfo: Add example support for NFS " David Howells
2020-02-21 20:21 ` [PATCH 00/17] VFS: Filesystem information and notifications " James Bottomley
2020-02-24 10:24   ` Miklos Szeredi
2020-02-24 14:55     ` James Bottomley
2020-02-24 15:28       ` Miklos Szeredi
2020-02-25 12:13         ` Steven Whitehouse
2020-02-25 15:28           ` James Bottomley [this message]
2020-02-25 15:47             ` Steven Whitehouse
2020-02-26  9:11             ` Miklos Szeredi
2020-02-26 10:51               ` Steven Whitehouse
2020-02-27  5:06               ` Ian Kent
2020-02-27  9:36                 ` Miklos Szeredi
2020-02-27 11:34                   ` Ian Kent
2020-02-27 13:45                     ` Miklos Szeredi
2020-02-27 15:14                       ` Karel Zak
2020-02-28  0:43                         ` Ian Kent
2020-02-28  8:35                           ` Miklos Szeredi
2020-02-28 12:27                             ` Greg Kroah-Hartman
2020-02-28 16:24                               ` Miklos Szeredi
2020-02-28 17:15                                 ` Al Viro
2020-03-02  8:43                                   ` Miklos Szeredi
2020-03-02 10:34                                 ` Karel Zak
2020-02-28 16:42                               ` David Howells
2020-02-28 15:08                             ` James Bottomley
2020-02-28 15:40                               ` Miklos Szeredi
2020-02-28  0:12                       ` Ian Kent
2020-02-28 15:52             ` Christian Brauner
2020-02-28 16:36             ` David Howells
2020-03-02  9:09               ` Miklos Szeredi
2020-03-02  9:38                 ` Greg Kroah-Hartman
2020-03-03  5:27                 ` Ian Kent
2020-03-03  7:46                   ` Miklos Szeredi
2020-03-06 16:25                     ` Miklos Szeredi
2020-03-06 19:43                       ` Al Viro
2020-03-06 19:54                         ` Miklos Szeredi
2020-03-06 19:58                         ` Al Viro
2020-03-06 20:05                           ` Al Viro
2020-03-06 20:11                             ` Miklos Szeredi
2020-03-06 20:37                             ` Al Viro
2020-03-06 20:38                               ` Al Viro
2020-03-06 20:45                                 ` Al Viro
2020-03-06 20:49                                   ` Al Viro
2020-03-06 20:51                                     ` Miklos Szeredi
2020-03-06 21:28                                       ` Al Viro
2020-03-06 20:56                                     ` Al Viro
2020-03-06 20:51                                   ` Miklos Szeredi
2020-03-07  9:48                       ` Greg Kroah-Hartman
2020-03-07 20:48                         ` Miklos Szeredi
2020-03-03  9:12                   ` David Howells
2020-03-03  9:26                     ` Miklos Szeredi
2020-03-03  9:48                       ` Miklos Szeredi
2020-03-03 10:21                         ` Steven Whitehouse
2020-03-03 10:32                           ` Miklos Szeredi
2020-03-03 11:09                             ` Ian Kent
2020-03-03 10:00                       ` Christian Brauner
2020-03-03 10:13                         ` Miklos Szeredi
2020-03-03 10:25                           ` Christian Brauner
2020-03-03 11:33                             ` Miklos Szeredi
2020-03-03 11:56                               ` Christian Brauner
2020-03-03 11:38                       ` Karel Zak
2020-03-03 13:03                         ` Greg Kroah-Hartman
2020-03-03 13:14                           ` Greg Kroah-Hartman
2020-03-03 13:34                             ` Miklos Szeredi
2020-03-03 13:43                               ` Greg Kroah-Hartman
2020-03-03 14:10                                 ` Greg Kroah-Hartman
2020-03-03 14:13                                   ` Jann Horn
2020-03-03 14:24                                     ` Greg Kroah-Hartman
2020-03-03 15:44                                       ` Jens Axboe
2020-03-03 16:37                                         ` Greg Kroah-Hartman
2020-03-03 16:51                                         ` Jeff Layton
2020-03-03 16:55                                           ` Jens Axboe
2020-03-03 19:02                                             ` Jeff Layton
2020-03-03 19:07                                               ` Jens Axboe
2020-03-03 19:23                                               ` Jens Axboe
2020-03-03 19:43                                                 ` Jeff Layton
2020-03-03 20:33                                                   ` Jens Axboe
2020-03-03 21:03                                                     ` Jeff Layton
2020-03-03 21:20                                                       ` Jens Axboe
2020-03-03 14:10                                 ` Miklos Szeredi
2020-03-03 14:29                                   ` Greg Kroah-Hartman
2020-03-03 14:40                                     ` Jann Horn
2020-03-03 16:51                                       ` Greg Kroah-Hartman
2020-03-03 16:57                                         ` Jann Horn
2020-03-03 20:15                                         ` Greg Kroah-Hartman
2020-03-03 14:40                                   ` David Howells
2020-03-04  4:20                                   ` Ian Kent
2020-03-03 14:19                                 ` David Howells
2020-03-03 16:59                                   ` Greg Kroah-Hartman
2020-03-03 14:23                               ` Christian Brauner
2020-03-03 15:23                                 ` Greg Kroah-Hartman
2020-03-03 15:53                                 ` David Howells
2020-03-04  2:01                           ` Ian Kent
2020-03-04 15:22                             ` Karel Zak
2020-03-04 16:49                               ` Greg Kroah-Hartman
2020-03-04 17:55                                 ` Karel Zak
2020-03-03 14:09                         ` David Howells

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