Re: file metadata via fs API

From: Al Viro <viro@zeniv.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Miklos Szeredi <miklos@szeredi.hu>,
	Steven Whitehouse <swhiteho@redhat.com>,
	David Howells <dhowells@redhat.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Karel Zak <kzak@redhat.com>, Jeff Layton <jlayton@redhat.com>,
	Miklos Szeredi <mszeredi@redhat.com>,
	Nicolas Dichtel <nicolas.dichtel@6wind.com>,
	Christian Brauner <christian@brauner.io>,
	Lennart Poettering <lennart@poettering.net>,
	Linux API <linux-api@vger.kernel.org>,
	Ian Kent <raven@themaw.net>,
	LSM <linux-security-module@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: file metadata via fs API
Date: Wed, 19 Aug 2020 03:29:07 +0100	[thread overview]
Message-ID: <20200819022907.GE1236603@ZenIV.linux.org.uk> (raw)
In-Reply-To: <CAHk-=wh5YifP7hzKSbwJj94+DZ2czjrZsczy6GBimiogZws=rg@mail.gmail.com>

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...