Re: [PATCH 00/19] VFS: Filesystem information and notifications [ver #16]

[Christian Brauner]

On Thu, Feb 20, 2020 at 12:42:15PM +0800, Ian Kent wrote:
> On Wed, 2020-02-19 at 15:46 +0100, Christian Brauner wrote:
> > On Tue, Feb 18, 2020 at 05:04:55PM +0000, David Howells wrote:
> > > Here are a set of patches that adds system calls, that (a) allow
> > > information about the VFS, mount topology, superblock and files to
> > > be
> > > retrieved and (b) allow for notifications of mount topology
> > > rearrangement
> > > events, mount and superblock attribute changes and other superblock
> > > events,
> > > such as errors.
> > > 
> > > ============================
> > > FILESYSTEM INFORMATION QUERY
> > > ============================
> > > 
> > > The first system call, fsinfo(), allows information about the
> > > filesystem at
> > > a particular path point to be queried as a set of attributes, some
> > > of which
> > > may have more than one value.
> > > 
> > > Attribute values are of four basic types:
> > > 
> > >  (1) Version dependent-length structure (size defined by type).
> > > 
> > >  (2) Variable-length string (up to 4096, including NUL).
> > > 
> > >  (3) List of structures (up to INT_MAX size).
> > > 
> > >  (4) Opaque blob (up to INT_MAX size).
> > 
> > I mainly have an organizational question. :) This is a huge patchset
> > with lots and lots of (good) features. Wouldn't it make sense to make
> > the fsinfo() syscall a completely separate patchset from the
> > watch_mount() and watch_sb() syscalls? It seems that they don't need
> > to
> > depend on each other at all. This would make reviewing this so much
> > nicer and likely would mean that fsinfo() could proceed a little
> > faster.
> 
> The remainder of the fsinfo() series would need to remain useful
> if this was done.
> 
> For context I want work on improving handling of large mount
> tables.

Yeah, I've talked to David about this; polling on a large mountinfo file
is not great, I agree.

> 
> Ultimately I expect to solve a very long standing autofs problem
> of using large direct mount maps without prohibitive performance
> overhead (and there a lot of rather challenging autofs changes to
> do for this too) and I believe the fsinfo() system call, and
> related bits, is the way to do this.
> 
> But improving the handling of large mount tables for autofs
> will have the side effect of improvements for other mount table
> users, even in the early stages of this work.
> 
> For example I want to use this for mount table handling improvements
> in libmount. Clearly that ultimately needs mount change notification
> in the end but ...
> 
> There's a bunch of things that need to be done alone the way
> to even get started.
> 
> One thing that's needed is the ability to call fsinfo() to get
> information on a mount to avoid constant reading of the proc based
> mount table, which happens a lot (since the mount info. needs
> to be up to date) so systemd (and others) would see an improvement
> with the fsinfo() system call alone able to be used in libmount.
> 
> But for the fsinfo() system call to be used for this the file
> system specific mount options need to also be obtained when
> using fsinfo(). That means the super block operation fsinfo uses
> to provide this must be implemented for at least most file systems.
> 
> So separating out the notifications part, leaving whatever is needed
> to still be able to do this, should be fine and the system call
> would be immediately useful once the super operation is implemented
> for the needed file systems.
> 
> Whether the implementation of the super operation should be done
> as part of this series is another question but would certainly
> be a challenge and make the series more complicated. But is needed
> for the change to be useful in my case.

I think what would might work - and what David had already brought up
briefly - is to either base the fsinfo branch on top of the mount
notificaiton branch or break the notification counters pieces into a
separate patch and base both mount notifications and fsinfo on top of
it.

Christian