linux-kernel - Re: [RFC] lsm: fs: Use i_callback to free i

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Message-ID: <CAPhsuW5Jik1B9SOXr992pQPDuaXbUuAw9Ktp_OYPEkaev5NdUA@mail.gmail.com>
Date: Tue, 17 Dec 2024 10:19:49 -0800
From: Song Liu <song@...nel.org>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org, 
	linux-xfs@...r.kernel.org, linux-security-module@...r.kernel.org, 
	willy@...radead.org, corbet@....net, clm@...com, josef@...icpanda.com, 
	dsterba@...e.com, brauner@...nel.org, jack@...e.cz, cem@...nel.org, 
	djwong@...nel.org, paul@...l-moore.com, jmorris@...ei.org, serge@...lyn.com, 
	fdmanana@...e.com, johannes.thumshirn@....com
Subject: Re: [RFC] lsm: fs: Use i_callback to free i_security in RCU callback

On Tue, Dec 17, 2024 at 9:38 AM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> >  - Let pipe free inode from a RCU callback.
>
> ... which hurts the systems with LSM crap disabled.
> NAK.

How do we measure the overhead in such cases? AFAICT,
the overhead is very small:

1. Many (most) systems have some LSM enabled anyway.
2. pipe create/release is not on any hot path. On a busy system
  with 176 CPUs, I measured ~30 pipe create/release per second.
3. The overhead of a rcu callback is small.

Given these measures, I don't think "hurts the system without LSM"
justifies 2 extra pointers per inode.

Thanks,
Song