Re: threads-max observe limits

[Eric W. Biederman]

Michal Hocko <mhocko@xxxxxxxxxx> writes:

> From 711000fdc243b6bc68a92f9ef0017ae495086d39 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@xxxxxxxx>
> Date: Sun, 22 Sep 2019 08:45:28 +0200
> Subject: [PATCH] kernel/sysctl.c: do not override max_threads provided by
>  userspace
>
> 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.
>
> Fixes: 16db3d3f1170 ("kernel/sysctl.c: threads-max observe limits")
> Cc: stable
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
Reviewed-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>

> ---
>  kernel/fork.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 2852d0e76ea3..ef865be37e98 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2929,7 +2929,7 @@ int sysctl_max_threads(struct ctl_table *table, int write,
>  	struct ctl_table t;
>  	int ret;
>  	int threads = max_threads;
> -	int min = MIN_THREADS;
> +	int min = 1;
>  	int max = MAX_THREADS;
>  
>  	t = *table;
> @@ -2941,7 +2941,7 @@ int sysctl_max_threads(struct ctl_table *table, int write,
>  	if (ret || !write)
>  		return ret;
>  
> -	set_max_threads(threads);
> +	max_threads = threads;
>  
>  	return 0;
>  }
> -- 
> 2.20.1