oom: move oom_adj value from task_struct to mm_struct

The per-task oom_adj value is a characteristic of its mm more than the
task itself since it's not possible to oom kill any thread that shares the
mm.  If a task were to be killed while attached to an mm that could not be
freed because another thread were set to OOM_DISABLE, it would have
needlessly been terminated since there is no potential for future memory
freeing.

This patch moves oomkilladj (now more appropriately named oom_adj) from
struct task_struct to struct mm_struct.  This requires task_lock() on a
task to check its oom_adj value to protect against exec, but it's already
necessary to take the lock when dereferencing the mm to find the total VM
size for the badness heuristic.

This fixes a livelock if the oom killer chooses a task and another thread
sharing the same memory has an oom_adj value of OOM_DISABLE.  This occurs
because oom_kill_task() repeatedly returns 1 and refuses to kill the
chosen task while select_bad_process() will repeatedly choose the same
task during the next retry.

Taking task_lock() in select_bad_process() to check for OOM_DISABLE and in
oom_kill_task() to check for threads sharing the same memory will be
removed in the next patch in this series where it will no longer be
necessary.

Writing to /proc/pid/oom_adj for a kthread will now return -EINVAL since
these threads are immune from oom killing already.  They simply report an
oom_adj value of OOM_DISABLE.

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, 16 insertions, 3 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 1539e630c47..3ce5ae9e3d2 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1006,7 +1006,12 @@ static ssize_t oom_adjust_read(struct file *file, char __user *buf,
 
 	if (!task)
 		return -ESRCH;
-	oom_adjust = task->oomkilladj;
+	task_lock(task);
+	if (task->mm)
+		oom_adjust = task->mm->oom_adj;
+	else
+		oom_adjust = OOM_DISABLE;
+	task_unlock(task);
 	put_task_struct(task);
 
 	len = snprintf(buffer, sizeof(buffer), "%i\n", oom_adjust);
@@ -1035,11 +1040,19 @@ static ssize_t oom_adjust_write(struct file *file, const char __user *buf,
 	task = get_proc_task(file->f_path.dentry->d_inode);
 	if (!task)
 		return -ESRCH;
-	if (oom_adjust < task->oomkilladj && !capable(CAP_SYS_RESOURCE)) {
+	task_lock(task);
+	if (!task->mm) {
+		task_unlock(task);
+		put_task_struct(task);
+		return -EINVAL;
+	}
+	if (oom_adjust < task->mm->oom_adj && !capable(CAP_SYS_RESOURCE)) {
+		task_unlock(task);
 		put_task_struct(task);
 		return -EACCES;
 	}
-	task->oomkilladj = oom_adjust;
+	task->mm->oom_adj = oom_adjust;
+	task_unlock(task);
 	put_task_struct(task);
 	if (end - buffer == 0)
 		return -EIO;