oom: avoid unnecessary mm locking and scanning for OOM_DISABLE

This moves the check for OOM_DISABLE to the badness heuristic so it is
only necessary to hold task_lock() once.  If the mm is OOM_DISABLE, the
score is 0, which is also correctly exported via /proc/pid/oom_score.
This requires that tasks with badness scores of 0 are prohibited from
being oom killed, which makes sense since they would not allow for future
memory freeing anyway.

Since the oom_adj value is a characteristic of an mm and not a task, it is
no longer necessary to check the oom_adj value for threads sharing the
same memory (except when simply issuing SIGKILLs for threads in other
thread groups).

Cc: Nick Piggin <npiggin@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 10 insertions, 32 deletions

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index b60913520ef3..cdcf89cb9ff2 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -67,6 +67,10 @@ unsigned long badness(struct task_struct *p, unsigned long uptime)
                 return 0;
         }
         oom_adj = mm->oom_adj;
+        if (oom_adj == OOM_DISABLE) {
+                task_unlock(p);
+                return 0;
+        }
 
         /*
          * The memory size of the process is the basis for the badness.
@@ -253,15 +257,8 @@ static struct task_struct *select_bad_process(unsigned long *ppoints,
                         *ppoints = ULONG_MAX;
                 }
 
-                task_lock(p);
-                if (p->mm && p->mm->oom_adj == OOM_DISABLE) {
-                        task_unlock(p);
-                        continue;
-                }
-                task_unlock(p);
-
                 points = badness(p, uptime.tv_sec);
-                if (points > *ppoints || !chosen) {
+                if (points > *ppoints) {
                         chosen = p;
                         *ppoints = points;
                 }
@@ -354,32 +351,13 @@ static int oom_kill_task(struct task_struct *p)
         struct mm_struct *mm;
         struct task_struct *g, *q;
 
+        task_lock(p);
         mm = p->mm;
-
-        /* WARNING: mm may not be dereferenced since we did not obtain its
-         * value from get_task_mm(p).  This is OK since all we need to do is
-         * compare mm to q->mm below.
-         *
-         * Furthermore, even if mm contains a non-NULL value, p->mm may
-         * change to NULL at any time since we do not hold task_lock(p).
-         * However, this is of no concern to us.
-         */
-
-        if (mm == NULL)
+        if (!mm || mm->oom_adj == OOM_DISABLE) {
+                task_unlock(p);
                 return 1;
-
-        /*
-         * Don't kill the process if any threads are set to OOM_DISABLE
-         */
-        do_each_thread(g, q) {
-                task_lock(q);
-                if (q->mm == mm && q->mm && q->mm->oom_adj == OOM_DISABLE) {
-                        task_unlock(q);
-                        return 1;
-                }
-                task_unlock(q);
-        } while_each_thread(g, q);
-
+        }
+        task_unlock(p);
         __oom_kill_task(p, 1);
 
         /*