linux-kernel - Re: [PATCH 01/11] sysfs: Support for preventing unmounts.

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Date:	Wed, 18 Jun 2008 13:12:27 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	Benjamin Thery <benjamin.thery@...l.net>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <htejun@...il.com>, linux-kernel@...r.kernel.org,
	Al Viro <viro@....linux.org.uk>,
	Linux Containers <containers@...ts.osdl.org>
Subject: Re: [PATCH 01/11] sysfs: Support for preventing unmounts.

Dave Hansen <dave@...ux.vnet.ibm.com> writes:

> On Wed, 2008-06-18 at 19:07 +0200, Benjamin Thery wrote:
>> To support mounting multiple instances of sysfs occassionally I
>> need to walk through all of the currently present sysfs super blocks.
>
> I know you may have addressed this before, but I forgot and it didn't
> make it into the changelogs.
>
> Why are you doing this again?  It seems like an awfully blunt
> instrument.  

So the fundamentals.
- The data in sysfs fundamentally changes behind the back of the
  VFS and we need to keep the VFS in sync.  Essentially this is the
  distributed filesystem problem.

- In particular for sysfs_rename and sysfs_move_dir we need to support finding
  the dcache entries and calling d_move.  So that the dcache does not
  get into an inconsistent state.  Timeouts and invalidates like NFS
  uses are to be  avoided if at all possible.

- Coming through the vfs we are guaranteed that the filesystem will
  not be unmounted while we have a reference on a dentry, and with
  multiple mounts we do not get that guarantee.  Therefore to get that
  guarantee for all of the superblocks we need the blunt instrument.

- Since mount/unmount are rare blocking them is no big deal.

I believe any distributed filesystem that is together enough to tell
us about renames (so we can update the dcache) instead of doing the
NFS timeout will need the ability to block mount/unmount while it is
executing d_move.

Currently sysfs does not need to block mounts only because we perform
an internal mount and then never unmount sysfs.

Eric
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