Re: [patch 0/6] vfs: mountinfo update

[Ram Pai <linuxram@us.ibm.com>]

On Fri, 2008-03-14 at 09:17 +0100, Miklos Szeredi wrote:
> > diffstat for all seven patches is:
> > 
> >  Documentation/filesystems/proc.txt |   35 +++++
> >  fs/dcache.c                        |  101 +++++++++++---
> >  fs/namespace.c                     |  182 ++++++++++++++++++++++-----
> >  fs/pnode.c                         |  131 ++++++++++++++++++-
> >  fs/pnode.h                         |   15 +-
> >  fs/proc/base.c                     |  121 +++++++++--------
> >  fs/seq_file.c                      |   92 +++++++++++--
> >  include/linux/dcache.h             |    5 
> >  include/linux/mnt_namespace.h      |   12 +
> >  include/linux/mount.h              |    2 
> >  include/linux/seq_file.h           |    7 +
> >  11 files changed, 559 insertions(+), 144 deletions(-)
> > 
> > that's a mountain of tricksy new core-kernel code just for some /proc file.
> > 
> > Is this all really justifiable?
> 
> This is an approximate order of added features from most complex to
> least complex:
> 
>  1) basic info about mount propagation
>  2) root of the mount (source directory of bind)
>  3) disambiguating reachable mounts from unreachable (/proc/mounts
>     under chroot is a mess, and people are afraid to fix it:
>     http://lkml.org/lkml/2007/4/17/271)
>  4) showing "dominating group" which intersects with current namespace/root
>  5) allocating ID's from IDR pools instead of ever increasing 64bit counters
>  6) showing ID of mount and parent mount
>  7) st_dev of mount
> 
> 1, 2, 3 and 6 we definitely want.  Al wants 4.  I think 5 is useful
> for readability, but not absolutely necessary.
> 
> All in all, I think there's very little that we can do, without
> throwing out the baby with the bathwater.
> 
> Also compare this with the number of lines and mind boggling
> complexity added for the mount propagation stuff, which is still very
> little used years after it's introduction.  Possibly because some of
> the infrastructure is still missing from kernel as well as userspace
> to make it all really useful.  And I think this patch set goes towards
> getting that infrastructure in place.

The inability to see the propagation tree was one major block towards
the use of sharedsubtree. Patch (1) fills up that gap and hence
absolutely needed. I view the remaining patches as patches directly
or indirectly supporting (1). 

As far as the 'mind boggling complex code' I believe the container folks
use it or at-least plan to use it. I know one our software groups
actively leveraging this functionality. 

RP


> 
> Possibly mount propagation will remain on the fringe for ever, in
> which case a big chunk of this patch set (1 and 4) also end up being
> useless.  The worst that can happen is that we end up adding some
> #ifdef CONFIG_MOUNT_PROPAGATION clutter to namespace.c.
> 
> Miklos