linux-ext4 - Re: [PATCH 00/13] VFS: Filesystem information [ver #19]

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Date:   Wed, 1 Apr 2020 10:37:22 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     David Howells <dhowells@...hat.com>
Cc:     Ian Kent <raven@...maw.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux NFS list <linux-nfs@...r.kernel.org>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Anna Schumaker <anna.schumaker@...app.com>,
        "Theodore Ts'o" <tytso@....edu>,
        Linux API <linux-api@...r.kernel.org>,
        linux-ext4@...r.kernel.org,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Christian Brauner <christian@...uner.io>,
        Jann Horn <jannh@...gle.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Karel Zak <kzak@...hat.com>, Jeff Layton <jlayton@...hat.com>,
        linux-fsdevel@...r.kernel.org,
        LSM <linux-security-module@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/13] VFS: Filesystem information [ver #19]

On Wed, Apr 1, 2020 at 10:27 AM David Howells <dhowells@...hat.com> wrote:
>
> Miklos Szeredi <miklos@...redi.hu> wrote:
>
> > According to dhowell's measurements processing 100k mounts would take
> > about a few seconds of system time (that's the time spent by the
> > kernel to retrieve the data,
>
> But the inefficiency of mountfs - at least as currently implemented - scales
> up with the number of individual values you want to retrieve, both in terms of
> memory usage and time taken.

I've taken that into account when guesstimating a "few seconds per
100k entries".  My guess is that there's probably an order of
magnitude difference between the performance of a fs based interface
and a binary syscall based interface.  That could be reduced somewhat
with a readfile(2) type API.

But the point is: this does not matter.  Whether it's .5s or 5s is
completely irrelevant, as neither is going to take down the system,
and userspace processing is probably going to take as much, if not
more time.  And remember, we are talking about stopping and starting
the automount daemon, which is something that happens, but it should
not happen often by any measure.

> With fsinfo(), I've tried to batch values together where it makes sense - and
> there's no lingering memory overhead - no extra inodes, dentries and files
> required.

The dentries, inodes and files in your test are single use (except the
root dentry) and can be made ephemeral if that turns out to be better.
My guess is that dentries belonging to individual attributes should be
deleted on final put, while the dentries belonging to the mount
directory can be reclaimed normally.

Thanks,
Miklos