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

[Ian Kent]

On Mon, 2020-03-02 at 10:09 +0100, Miklos Szeredi wrote:
> On Fri, Feb 28, 2020 at 5:36 PM David Howells <dhowells@xxxxxxxxxx>
> wrote:
> > sysfs also has some other disadvantages for this:
> > 
> >  (1) There's a potential chicken-and-egg problem in that you have
> > to create a
> >      bunch of files and dirs in sysfs for every created mount and
> > superblock
> >      (possibly excluding special ones like the socket mount) - but
> > this
> >      includes sysfs itself.  This might work - provided you create
> > sysfs
> >      first.
> 
> Sysfs architecture looks something like this (I hope Greg will
> correct
> me if I'm wrong):
> 
> device driver -> kobj tree <- sysfs tree
> 
> The kobj tree is created by the device driver, and the dentry tree is
> created on demand from the kobj tree.   Lifetime of kobjs is bound to
> both the sysfs objects and the device but not the other way round.
> I.e. device can go away while the sysfs object is still being
> referenced, and sysfs can be freely mounted and unmounted
> independently of device initialization.
> 
> So there's no ordering requirement between sysfs mounts and other
> mounts.   I might be wrong on the details, since mounts are created
> very early in the boot process...
> 
> >  (2) sysfs is memory intensive.  The directory structure has to be
> > backed by
> >      dentries and inodes that linger as long as the referenced
> > object does
> >      (procfs is more efficient in this regard for files that aren't
> > being
> >      accessed)
> 
> See above: I don't think dentries and inodes are pinned, only kobjs
> and their associated cruft.  Which may be too heavy, depending on the
> details of the kobj tree.
> 
> >  (3) It gives people extra, indirect ways to pin mount objects and
> >      superblocks.
> 
> See above.
> 
> > For the moment, fsinfo() gives you three ways of referring to a
> > filesystem
> > object:
> > 
> >  (a) Directly by path.
> 
> A path is always representable by an O_PATH descriptor.
> 
> >  (b) By path associated with an fd.
> 
> See my proposal about linking from /proc/$PID/fdmount/$FD ->
> /sys/devices/virtual/mounts/$MOUNT_ID.
> 
> >  (c) By mount ID (perm checked by working back up the tree).
> 
> Check that perm on lookup of /sys/devices/virtual/mounts/$MOUNT_ID.
> The proc symlink would bypass the lookup check by directly jumping to
> the mountinfo dir.
> 
> > but will need to add:
> > 
> >  (d) By fscontext fd (which is hard to find in sysfs).  Indeed, the
> > superblock
> >      may not even exist yet.
> 
> Proc symlink would work for that too.

There's mounts enumeration too, ordering is required to identify the
top (or bottom depending on terminology) with more than one mount on
a mount point.

> 
> If sysfs is too heavy, this could be proc or a completely new
> filesystem.  The implementation is much less relevant at this stage
> of
> the discussion than the interface.

Ha, proc with the seq file interface, that's already proved to not
work properly and looks difficult to fix.

Ian