[RFC PATCH 4/6] mm, oom: Use mm_ref()/mm_unref() and avoid mmdrop_async()

[Jann Horn]

The OOM killer uses MMF_OOM_SKIP to avoid running on an mm that has started
__mmput(); it only uses the mmgrab() reference to ensure that the mm_struct
itself stays alive.

This means that we don't need a full mmgrab() reference, which will keep
the pgd (and potentially also some pmd pages) alive and can't be cleaned up
from RCU callback context; we can use an mm_ref() reference instead.

Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
 kernel/fork.c | 6 +-----
 mm/oom_kill.c | 2 +-
 2 files changed, 2 insertions(+), 6 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index fcdd1ace79e4..59c119b03351 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -686,12 +686,8 @@ static inline void free_signal_struct(struct signal_struct *sig)
 {
 	taskstats_tgid_free(sig);
 	sched_autogroup_exit(sig);
-	/*
-	 * __mmdrop is not safe to call from softirq context on x86 due to
-	 * pgd_dtor so postpone it to the async context
-	 */
 	if (sig->oom_mm)
-		mmdrop_async(sig->oom_mm);
+		mm_unref(sig->oom_mm);
 	kmem_cache_free(signal_cachep, sig);
 }
 
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index e90f25d6385d..12967f54fbcf 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -704,7 +704,7 @@ static void mark_oom_victim(struct task_struct *tsk)
 
 	/* oom_mm is bound to the signal struct life time. */
 	if (!cmpxchg(&tsk->signal->oom_mm, NULL, mm)) {
-		mmgrab(tsk->signal->oom_mm);
+		mm_ref(tsk->signal->oom_mm);
 		set_bit(MMF_OOM_VICTIM, &mm->flags);
 	}
 
-- 
2.29.0.rc1.297.gfa9743e501-goog