linux-kernel - Re: [PATCH v4] fs/namespace: defer RCU sync for MNT

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <mey7l4rm7r5fxndlg72jfjjwwctyoimjg35jetrnv5gbee4qll@w5ldyvm6h22a>
Date: Wed, 9 Apr 2025 15:04:43 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: Eric Chanudet <echanude@...hat.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Clark Williams <clrkwllms@...nel.org>, 
	Steven Rostedt <rostedt@...dmis.org>, Ian Kent <ikent@...hat.com>, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev, 
	Alexander Larsson <alexl@...hat.com>, Lucas Karpinski <lkarpins@...hat.com>
Subject: Re: [PATCH v4] fs/namespace: defer RCU sync for MNT_DETACH umount

On Tue, Apr 08, 2025 at 04:58:34PM -0400, Eric Chanudet wrote:
> Defer releasing the detached file-system when calling namespace_unlock()
> during a lazy umount to return faster.
> 
> When requesting MNT_DETACH, the caller does not expect the file-system
> to be shut down upon returning from the syscall. Calling
> synchronize_rcu_expedited() has a significant cost on RT kernel that
> defaults to rcupdate.rcu_normal_after_boot=1. Queue the detached struct
> mount in a separate list and put it on a workqueue to run post RCU
> grace-period.
> 
> w/o patch, 6.15-rc1 PREEMPT_RT:
> perf stat -r 10 --null --pre 'mount -t tmpfs tmpfs mnt' -- umount mnt
>     0.02455 +- 0.00107 seconds time elapsed  ( +-  4.36% )
> perf stat -r 10 --null --pre 'mount -t tmpfs tmpfs mnt' -- umount -l mnt
>     0.02555 +- 0.00114 seconds time elapsed  ( +-  4.46% )
> 
> w/ patch, 6.15-rc1 PREEMPT_RT:
> perf stat -r 10 --null --pre 'mount -t tmpfs tmpfs mnt' -- umount mnt
>     0.026311 +- 0.000869 seconds time elapsed  ( +-  3.30% )
> perf stat -r 10 --null --pre 'mount -t tmpfs tmpfs mnt' -- umount -l mnt
>     0.003194 +- 0.000160 seconds time elapsed  ( +-  5.01% )
> 

Christian wants the patch done differently and posted his diff, so I'm
not going to comment on it.

I do have some feedback about the commit message though.

In v1 it points out a real user which runs into it, while this one does
not. So I would rewrite this and put in bench results from the actual
consumer -- as it is one is left to wonder why patching up lazy unmount
is of any significance.

I had to look up what rcupdate.rcu_normal_after_boot=1 is. Docs claim it
makes everyone use normal grace-periods, which explains the difference.
But without that one is left to wonder if perhaps there is a perf bug in
RCU instead where this is taking longer than it should despite the
option. Thus I would also denote how the delay shows up.

v1 for reference:
> v1: https://lore.kernel.org/all/20230119205521.497401-1-echanude@redhat.com/