linux-kernel - Re: [PATCH 4.19 56/81] kernel/sysctl.c: do not override max

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Message-ID: <20191017105940.GA5966@amd>
Date:   Thu, 17 Oct 2019 12:59:40 +0200
From:   Pavel Machek <pavel@...x.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Michal Hocko <mhocko@...e.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Heinrich Schuchardt <xypron.glpk@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 4.19 56/81] kernel/sysctl.c: do not override max_threads
 provided by userspace

Hi!

> From: Michal Hocko <mhocko@...e.com>
> 
> commit b0f53dbc4bc4c371f38b14c391095a3bb8a0bb40 upstream.
> 
> Partially revert 16db3d3f1170 ("kernel/sysctl.c: threads-max observe
> limits") because the patch is causing a regression to any workload which
> needs to override the auto-tuning of the limit provided by kernel.
> 
> set_max_threads is implementing a boot time guesstimate to provide a
> sensible limit of the concurrently running threads so that runaways will
> not deplete all the memory.  This is a good thing in general but there
> are workloads which might need to increase this limit for an application
> to run (reportedly WebSpher MQ is affected) and that is simply not
> possible after the mentioned change.  It is also very dubious to
> override an admin decision by an estimation that doesn't have any direct
> relation to correctness of the kernel operation.
> 
> Fix this by dropping set_max_threads from sysctl_max_threads so any
> value is accepted as long as it fits into MAX_THREADS which is important
> to check because allowing more threads could break internal robust futex
> restriction.  While at it, do not use MIN_THREADS as the lower boundary
> because it is also only a heuristic for automatic estimation and admin
> might have a good reason to stop new threads to be created even when
> below this limit.

Ok, why not, but I smell followup work could be done:

> @@ -2635,7 +2635,7 @@ int sysctl_max_threads(struct ctl_table
>  	if (ret || !write)
>  		return ret;
>  
> -	set_max_threads(threads);
> +	max_threads = threads;
>  

AFAICT set_max_threads can now become __init.

Plus, I don't see any locking here, should this be WRITE_ONCE() at
minimum?

Best regards,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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