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>

2 files changed, 2 insertions, 1 deletions

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 0e80e26ecf21..f4408106fcbc 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -232,6 +232,8 @@ struct mm_struct {
 
         unsigned long saved_auxv[AT_VECTOR_SIZE]; /* for /proc/PID/auxv */
 
+        s8 oom_adj;     /* OOM kill score adjustment (bit shift) */
+
         cpumask_t cpu_vm_mask;
 
         /* Architecture-specific MM context */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 1048bf50540a..1bc6fae0c135 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1178,7 +1178,6 @@ struct task_struct {
          * a short time
          */
         unsigned char fpu_counter;
-        s8 oomkilladj; /* OOM kill score adjustment (bit shift). */
 #ifdef CONFIG_BLK_DEV_IO_TRACE
         unsigned int btrace_seq;
 #endif