oom: add per-mm oom disable count

It's pointless to kill a task if another thread sharing its mm cannot be
killed to allow future memory freeing.  A subsequent patch will prevent
kills in such cases, but first it's necessary to have a way to flag a task
that shares memory with an OOM_DISABLE task that doesn't incur an
additional tasklist scan, which would make select_bad_process() an O(n^2)
function.

This patch adds an atomic counter to struct mm_struct that follows how
many threads attached to it have an oom_score_adj of OOM_SCORE_ADJ_MIN.
They cannot be killed by the kernel, so their memory cannot be freed in
oom conditions.

This only requires task_lock() on the task that we're operating on, it
does not require mm->mmap_sem since task_lock() pins the mm and the
operation is atomic.

[rientjes@google.com: changelog and sys_unshare() code]
[rientjes@google.com: protect oom_disable_count with task_lock in fork]
[rientjes@google.com: use old_mm for oom_disable_count in exec]
Signed-off-by: Ying Han <yinghan@google.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 35 insertions, 0 deletions

diff --git a/fs/exec.c b/fs/exec.c
index 6d2b6f936858..3aa75b8888a1 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -54,6 +54,7 @@
 #include <linux/fsnotify.h>
 #include <linux/fs_struct.h>
 #include <linux/pipe_fs_i.h>
+#include <linux/oom.h>
 
 #include <asm/uaccess.h>
 #include <asm/mmu_context.h>
@@ -759,6 +760,10 @@ static int exec_mmap(struct mm_struct *mm)
         tsk->mm = mm;
         tsk->active_mm = mm;
         activate_mm(active_mm, mm);
+        if (old_mm && tsk->signal->oom_score_adj == OOM_SCORE_ADJ_MIN) {
+                atomic_dec(&old_mm->oom_disable_count);
+                atomic_inc(&tsk->mm->oom_disable_count);
+        }
         task_unlock(tsk);
         arch_pick_mmap_layout(mm);
         if (old_mm) {
diff --git a/fs/proc/base.c b/fs/proc/base.c
index dc5d5f51f3fe..6e50c8e65513 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1047,6 +1047,21 @@ static ssize_t oom_adjust_write(struct file *file, const char __user *buf,
                 return -EACCES;
         }
 
+        task_lock(task);
+        if (!task->mm) {
+                task_unlock(task);
+                unlock_task_sighand(task, &flags);
+                put_task_struct(task);
+                return -EINVAL;
+        }
+
+        if (oom_adjust != task->signal->oom_adj) {
+                if (oom_adjust == OOM_DISABLE)
+                        atomic_inc(&task->mm->oom_disable_count);
+                if (task->signal->oom_adj == OOM_DISABLE)
+                        atomic_dec(&task->mm->oom_disable_count);
+        }
+
         /*
          * Warn that /proc/pid/oom_adj is deprecated, see
          * Documentation/feature-removal-schedule.txt.
@@ -1065,6 +1080,7 @@ static ssize_t oom_adjust_write(struct file *file, const char __user *buf,
         else
                 task->signal->oom_score_adj = (oom_adjust * OOM_SCORE_ADJ_MAX) /
                                                                 -OOM_DISABLE;
+        task_unlock(task);
         unlock_task_sighand(task, &flags);
         put_task_struct(task);
 
@@ -1133,6 +1149,19 @@ static ssize_t oom_score_adj_write(struct file *file, const char __user *buf,
                 return -EACCES;
         }
 
+        task_lock(task);
+        if (!task->mm) {
+                task_unlock(task);
+                unlock_task_sighand(task, &flags);
+                put_task_struct(task);
+                return -EINVAL;
+        }
+        if (oom_score_adj != task->signal->oom_score_adj) {
+                if (oom_score_adj == OOM_SCORE_ADJ_MIN)
+                        atomic_inc(&task->mm->oom_disable_count);
+                if (task->signal->oom_score_adj == OOM_SCORE_ADJ_MIN)
+                        atomic_dec(&task->mm->oom_disable_count);
+        }
         task->signal->oom_score_adj = oom_score_adj;
         /*
          * Scale /proc/pid/oom_adj appropriately ensuring that OOM_DISABLE is
@@ -1143,6 +1172,7 @@ static ssize_t oom_score_adj_write(struct file *file, const char __user *buf,
         else
                 task->signal->oom_adj = (oom_score_adj * OOM_ADJUST_MAX) /
                                                         OOM_SCORE_ADJ_MAX;
+        task_unlock(task);
         unlock_task_sighand(task, &flags);
         put_task_struct(task);
         return count;