[PATCH] proc, oom: do not report alien mms when setting oom_score_adj

From: Michal Hocko
Date: Tue Feb 12 2019 - 05:21:41 EST

From: Michal Hocko <mhocko@xxxxxxxx>

Tetsuo has reported that creating a thousands of processes sharing MM
without SIGHAND (aka alien threads) and setting
/proc/<pid>/oom_score_adj will swamp the kernel log and takes ages [1]
to finish. This is especially worrisome that all that printing is done
under RCU lock and this can potentially trigger RCU stall or softlockup

The primary reason for the printk was to catch potential users who might
depend on the behavior prior to 44a70adec910 ("mm, oom_adj: make sure
processes sharing mm have same view of oom_score_adj") but after more
than 2 years without a single report I guess it is safe to simply remove
the printk altogether.

The next step should be moving oom_score_adj over to the mm struct and
remove all the tasks crawling as suggested by [2]

[1] http://lkml.kernel.org/r/97fce864-6f75-bca5-14bc-12c9f890e740@xxxxxxxxxxxxxxxxxxx
[2] http://lkml.kernel.org/r/20190117155159.GA4087@xxxxxxxxxxxxxx
Reported-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
fs/proc/base.c | 4 ----
1 file changed, 4 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 633a63462573..f5ed9512d193 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1086,10 +1086,6 @@ static int __set_oom_adj(struct file *file, int oom_adj, bool legacy)

if (!p->vfork_done && process_shares_mm(p, mm)) {
- pr_info("updating oom_score_adj for %d (%s) from %d to %d because it shares mm with %d (%s). Report if this is unexpected.\n",
- task_pid_nr(p), p->comm,
- p->signal->oom_score_adj, oom_adj,
- task_pid_nr(task), task->comm);
p->signal->oom_score_adj = oom_adj;
if (!legacy && has_capability_noaudit(current, CAP_SYS_RESOURCE))
p->signal->oom_score_adj_min = (short)oom_adj;