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SubjectRe: pids.current with invalid value for hours [5.0.0 rc3 git]
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Arkadiusz, will you try this patch?

From 48744b6339cf649a69b55997e138c17df1ecc897 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Date: Sat, 26 Jan 2019 20:00:51 +0900
Subject: [PATCH] oom, oom_reaper: do not enqueue same task twice

Arkadiusz reported that enabling memcg's group oom killing causes
strange memcg statistics where there is no task in a memcg despite
the number of tasks in that memcg is not 0. It turned out that there
is a bug in wake_oom_reaper() which allows enqueuing same task twice
which makes impossible to decrease the number of tasks in that memcg
due to a refcount leak.

This bug existed since the OOM reaper became invokable from
task_will_free_mem(current) path in out_of_memory() in Linux 4.7,
but memcg's group oom killing made it easier to trigger this bug by
calling wake_oom_reaper() on the same task from one out_of_memory()
request.

Fix this bug using an approach used by commit 855b018325737f76
("oom, oom_reaper: disable oom_reaper for oom_kill_allocating_task").
Since task_will_free_mem(p) == false if p->mm == NULL, we can assume that
p->mm != NULL when wake_oom_reaper() is called from task_will_free_mem()
paths. As a side effect of this patch, this patch also avoids enqueuing
multiple threads sharing memory via task_will_free_mem(current) path.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: Arkadiusz Miśkiewicz <a.miskiewicz@gmail.com>
Fixes: af8e15cc85a25315 ("oom, oom_reaper: do not enqueue task if it is on the oom_reaper_list head")
---
mm/oom_kill.c | 28 +++++++++++++---------------
1 file changed, 13 insertions(+), 15 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index f0e8cd9..457f240 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -505,14 +505,6 @@ bool __oom_reap_task_mm(struct mm_struct *mm)
struct vm_area_struct *vma;
bool ret = true;

- /*
- * Tell all users of get_user/copy_from_user etc... that the content
- * is no longer stable. No barriers really needed because unmapping
- * should imply barriers already and the reader would hit a page fault
- * if it stumbled over a reaped memory.
- */
- set_bit(MMF_UNSTABLE, &mm->flags);
-
for (vma = mm->mmap ; vma; vma = vma->vm_next) {
if (!can_madv_dontneed_vma(vma))
continue;
@@ -645,10 +637,15 @@ static int oom_reaper(void *unused)
return 0;
}

-static void wake_oom_reaper(struct task_struct *tsk)
+static void wake_oom_reaper(struct task_struct *tsk, struct mm_struct *mm)
{
- /* tsk is already queued? */
- if (tsk == oom_reaper_list || tsk->oom_reaper_list)
+ /*
+ * Tell all users of get_user/copy_from_user etc... that the content
+ * is no longer stable. No barriers really needed because unmapping
+ * should imply barriers already and the reader would hit a page fault
+ * if it stumbled over a reaped memory.
+ */
+ if (test_and_set_bit(MMF_UNSTABLE, &mm->flags))
return;

get_task_struct(tsk);
@@ -668,7 +665,8 @@ static int __init oom_init(void)
}
subsys_initcall(oom_init)
#else
-static inline void wake_oom_reaper(struct task_struct *tsk)
+static inline void wake_oom_reaper(struct task_struct *tsk,
+ struct mm_struct *mm)
{
}
#endif /* CONFIG_MMU */
@@ -915,7 +913,7 @@ static void __oom_kill_process(struct task_struct *victim)
rcu_read_unlock();

if (can_oom_reap)
- wake_oom_reaper(victim);
+ wake_oom_reaper(victim, mm);

mmdrop(mm);
put_task_struct(victim);
@@ -955,7 +953,7 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
task_lock(p);
if (task_will_free_mem(p)) {
mark_oom_victim(p);
- wake_oom_reaper(p);
+ wake_oom_reaper(p, p->mm);
task_unlock(p);
put_task_struct(p);
return;
@@ -1085,7 +1083,7 @@ bool out_of_memory(struct oom_control *oc)
*/
if (task_will_free_mem(current)) {
mark_oom_victim(current);
- wake_oom_reaper(current);
+ wake_oom_reaper(current, current->mm);
return true;
}

--
1.8.3.1
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