oom, oom_reaper: allow to reap mm shared by the kthreads

oom reaper was skipped for an mm which is shared with the kernel thread
(aka use_mm()).  The primary concern was that such a kthread might want to
read from the userspace memory and see zero page as a result of the oom
reaper action.  This is no longer a problem after "mm: make sure that
kthreads will not refault oom reaped memory" because any attempt to fault
in when the MMF_UNSTABLE is set will result in SIGBUS and so the target
user should see an error.  This means that we can finally allow oom reaper
also to tasks which share their mm with kthreads.

Link: http://lkml.kernel.org/r/1472119394-11342-10-git-send-email-mhocko@kernel.org
Signed-off-by: Michal Hocko <mhocko@suse.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Vladimir Davydov <vdavydov@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 5a3ba96c8338495eb78ccd688fc87fd933d5f437..10f686969fc4afd708652459fd34d854e30da755 100644 (file)
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -902,13 +902,7 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
                        continue;
                if (same_thread_group(p, victim))
                        continue;
-               if (unlikely(p->flags & PF_KTHREAD) || is_global_init(p)) {
-                       /*
-                        * We cannot use oom_reaper for the mm shared by this
-                        * process because it wouldn't get killed and so the
-                        * memory might be still used. Hide the mm from the oom
-                        * killer to guarantee OOM forward progress.
-                        */
+               if (is_global_init(p)) {
                        can_oom_reap = false;
                        set_bit(MMF_OOM_SKIP, &mm->flags);
                        pr_info("oom killer %d (%s) has mm pinned by %d (%s)\n",
@@ -916,6 +910,12 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
                                        task_pid_nr(p), p->comm);
                        continue;
                }
+               /*
+                * No use_mm() user needs to read from the userspace so we are
+                * ok to reap it.
+                */
+               if (unlikely(p->flags & PF_KTHREAD))
+                       continue;
                do_send_sig_info(SIGKILL, SEND_SIG_FORCED, p, true);
        }
        rcu_read_unlock();