Re: Upcoming: Notifications, FS notifications and fsinfo()

[Miklos Szeredi <miklos@szeredi.hu>]

On Thu, Apr 2, 2020 at 4:52 AM Ian Kent <raven@themaw.net> wrote:
>
> On Wed, 2020-04-01 at 18:40 +0200, Miklos Szeredi wrote:
> > On Wed, Apr 1, 2020 at 6:07 PM David Howells <dhowells@redhat.com>
> > wrote:
> > > Miklos Szeredi <miklos@szeredi.hu> wrote:
> > >
> > > > I've still not heard a convincing argument in favor of a syscall.
> > >
> > > From your own results, scanning 10000 mounts through mountfs and
> > > reading just
> > > two values from each is an order of magnitude slower without the
> > > effect of the
> > > dentry/inode caches.  It gets faster on the second run because the
> > > mountfs
> > > dentries and inodes are cached - but at a cost of >205MiB of
> > > RAM.  And it's
> > > *still* slower than fsinfo().
> >
> > Already told you that we can just delete the dentry on dput_final, so
> > the memory argument is immaterial.
> >
> > And the speed argument also, because there's no use case where that
> > would make a difference.  You keep bringing up the notification queue
> > overrun when watching a subtree, but that's going to be painful with
> > fsinfo(2) as well.   If that's a relevant use case (not saying it's
> > true), might as well add a /mnt/MNT_ID/subtree_info (trivial again)
> > that contains all information for the subtree.  Have fun implementing
> > that with fsinfo(2).
>
> Forgive me for not trawling through your patch to work this out
> but how does a poll on a path get what's needed to get mount info.
>
> Or, more specifically, how does one get what's needed to go directly
> to the place to get mount info. when something in the tree under the
> polled path changes (mount/umount). IIUC poll alone won't do subtree
> change monitoring?

The mechanisms are basically the same as with fsinfo(2).   You can get
to the mountfs entry through the mount ID or through a proc/fd/ type
symlink.  So if you have a path, there are two options:

 - find out the mount ID belonging to that path and go to /mountfs/$mntid/
 - open the path with fd = open(path, O_PATH) and the go to
/proc/self/fdmount/$fd/

Currently the only way to find the mount id from a path is by parsing
/proc/self/fdinfo/$fd.  It is trivial, however, to extend statx(2) to
return it directly from a path.   Also the mount notification queue
that David implemented contains the mount ID of the changed mount.

> Don't get me wrong, neither the proc nor the fsinfo implementations
> deal with the notification storms that cause much of the problem we
> see now.
>
> IMHO that's a separate and very difficult problem in itself that
> can't even be considered until getting the information efficiently
> is resolved.

This mount notification storm issue got me thinking.   If I understand
correctly, systemd wants mount notifications so that it can do the
desktop pop-up thing.   Is that correct?

But that doesn't apply to automounts at all.  A new mount performed by
automount is uninteresting to to desktops, since it's triggered by
crossing the automount point (i.e. a normal path lookup), not an
external event like inserting a usb stick, etc...

Am I missing something?

Maybe the solution is to just allow filtering out such notifications
at the source, so automount triggers don't generate events for
systemd.

Thanks,
Miklos