linux-kernel - Re: Upcoming: Notifications, FS notifications and fsinfo()

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Date:   Wed, 1 Apr 2020 17:25:54 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     David Howells <dhowells@...hat.com>
Cc:     Karel Zak <kzak@...hat.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>, dray@...hat.com,
        Miklos Szeredi <mszeredi@...hat.com>,
        Steven Whitehouse <swhiteho@...hat.com>,
        Jeff Layton <jlayton@...hat.com>, Ian Kent <raven@...maw.net>,
        andres@...razel.de, keyrings@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Lennart Poettering <lennart@...ttering.net>,
        Aleksa Sarai <cyphar@...har.com>
Subject: Re: Upcoming: Notifications, FS notifications and fsinfo()

On Wed, Apr 1, 2020 at 3:58 PM David Howells <dhowells@...hat.com> wrote:
>
> David Howells <dhowells@...hat.com> wrote:
>
> > > Attached patch applies against readfile patch.
> >
> > But doesn't actually do what Karel asked for.  show_mountinfo() itself does
> > not give you what Karel asked for.

Not sure what you mean.  I think it shows precisely the information
Karel asked for.

>  Plus there's more information you need to
> > add to it.

The mountinfo format is extensible (see
Documentation/filesystems/proc.txt) so for example adding the change
counters would be simple.

> And arguably, it's worse than just reading /proc/mounts.  If you get a
> notification that something changed (ie. you poll /proc/mounts or mount
> notifications gives you an overrun) you now have to read *every*
> /mountfs/*/info file.  That is way more expensive.

fsinfo(2) will never be substantially cheaper than reading and parsing
/mnt/MNT_ID/info.  In fact reading a large part of the mount table
using fsinfo(2) will be substantially slower than parsing
/proc/self/mountinfo (this doesn't actually do the parsing but that
would add a very small amount of overhead):

root@kvm:~# ./test-fsinfo-perf /tmp/a 30000
--- make mounts ---
--- test fsinfo by path ---
sum(mnt_id) = 960000
--- test fsinfo by mnt_id ---
sum(mnt_id) = 960000
--- test /proc/fdinfo ---
sum(mnt_id) = 960000
--- test mountfs ---
sum(mnt_id) = 960000
--- test mountinfo ---
sum(mnt_id) = 960000
For   30000 mounts, f=    154963us f2=    148337us p=   1803699us p2=
  257019us; m=     53996us; p=11.6*f p=12.2*f2 p=7.0*p2 p=33.4*m
--- umount ---

Yes, that's 33 times faster!

Thanks,
Miklos

View attachment "test-fsinfo-perf-mountinfo.patch" of type "text/x-patch" (3709 bytes)