[PATCH] kernel/sysctl.c: do not override max_threads provided by userspace

[Michal Hocko]

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>
---
 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

-- 
Michal Hocko
SUSE Labs