[PATCH] oom, oom_reaper: allow to reap mm shared by the kthreads

[Michal Hocko]

oom reaper was skipped for an mm which is shared with the kernel thread
(aka use_mm()). The primary concern was that such a kthread might want
to read from the userspace memory and see zero page as a result of the
oom reaper action. This seems to be overly conservative because none of
the current use_mm() users need to do copy_from_user or get_user. aio
code used to rely on copy_from_user but this is long gone along with
use_mm() usage in fs/aio.c.

We currently have only 3 users in the kernel:
- ffs_user_copy_worker, ep_user_copy_worker only do copy_to_iter()
- vhost_worker only copies over to the userspace as well AFAICS

In fact relying on copy_from_user in the kernel thread context is quite
dubious because it expects an active cooperation from the userspace to
have a consistent data (e.g. userspace can do MADV_DONTNEED as well).

Add a note to use_mm about the copy_from_user risk and allow the oom
killer to invoke the oom_reaper for mms shared with kthreads. This will
practically cause all the sane use cases to be reapable.

Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 mm/mmu_context.c |  5 +++++
 mm/oom_kill.c    | 14 +++++++-------
 2 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/mm/mmu_context.c b/mm/mmu_context.c
index f802c2d216a7..27449747f8de 100644
--- a/mm/mmu_context.c
+++ b/mm/mmu_context.c
@@ -16,6 +16,11 @@
  *	mm context.
  *	(Note: this routine is intended to be called only
  *	from a kernel thread context)
+ *
+ *	Do not use copy_from_user from this context because the
+ *	address space might got reclaimed behind the back by
+ *	the oom_reaper so an unexpected zero page might be
+ *	encountered.
  */
 void use_mm(struct mm_struct *mm)
 {
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 6303bc7caeda..b6a7027643b6 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -921,13 +921,7 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
 			continue;
 		if (same_thread_group(p, victim))
 			continue;
-		if (unlikely(p->flags & PF_KTHREAD) || is_global_init(p)) {
-			/*
-			 * We cannot use oom_reaper for the mm shared by this
-			 * process because it wouldn't get killed and so the
-			 * memory might be still used. Hide the mm from the oom
-			 * killer to guarantee OOM forward progress.
-			 */
+		if (is_global_init(p)) {
 			can_oom_reap = false;
 			set_bit(MMF_OOM_REAPED, &mm->flags);
 			pr_info("oom killer %d (%s) has mm pinned by %d (%s)\n",
@@ -935,6 +929,12 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
 					task_pid_nr(p), p->comm);
 			continue;
 		}
+		/*
+		 * No use_mm() user needs to read from the userspace so we are
+		 * ok to reap it.
+		 */
+		if (unlikely(p->flags & PF_KTHREAD))
+			continue;
 		do_send_sig_info(SIGKILL, SEND_SIG_FORCED, p, true);
 	}
 	rcu_read_unlock();
-- 
2.8.1

-- 
Michal Hocko
SUSE Labs