linux-kernel - Re: file metadata via fs API

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Date:   Wed, 19 Aug 2020 03:29:07 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Miklos Szeredi <miklos@...redi.hu>,
        Steven Whitehouse <swhiteho@...hat.com>,
        David Howells <dhowells@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Karel Zak <kzak@...hat.com>, Jeff Layton <jlayton@...hat.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        Christian Brauner <christian@...uner.io>,
        Lennart Poettering <lennart@...ttering.net>,
        Linux API <linux-api@...r.kernel.org>,
        Ian Kent <raven@...maw.net>,
        LSM <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: file metadata via fs API

On Tue, Aug 18, 2020 at 11:51:25AM -0700, Linus Torvalds wrote:

> I think people who have problems parsing plain ASCII text are just
> wrong. It's not that expensive. The thing that makes /proc/mounts
> expensive is not the individual lines - it's that there are a lot of
> them.

	It is expensive - if you use strdup() all over the place,
do asprintf() equivalents for concatenation, etc.  IOW, you can write
BASIC (or javascript) in any language...

	systemd used to be that bad - exactly in parsing /proc/mounts;
I hadn't checked that code lately, so it's possible that it had gotten
better, but about 4 years ago it had been awful.  OTOH, at that time
I'd been looking at the atrocities kernel-side (in fs/pnode.c), where
on realistic setups we had O(N^2) allocations done, with all but O(N)
of them ending up freed before anyone could see them.  So it's not as
if they had a monopoly on bloody awful code...