[PATCH 2/2] memcg: mm_update_next_owner: move for_each_thread() into try_to_set_owner()

From: Oleg Nesterov
Date: Wed Jun 26 2024 - 11:31:33 EST


mm_update_next_owner() checks the children / real_parent->children to
avoid the "everything else" loop in the likely case, but this won't work
if a child/sibling has a zombie leader with ->mm == NULL.

Move the for_each_thread() logic into try_to_set_owner(), if nothing else
this makes the children/siblings/everything searches more consistent.

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---
kernel/exit.c | 40 ++++++++++++++++++++++++----------------
1 file changed, 24 insertions(+), 16 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index a1ef5f23d5be..cc56edc1103e 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -440,7 +440,7 @@ static void coredump_task_exit(struct task_struct *tsk)

#ifdef CONFIG_MEMCG
/* drops tasklist_lock if succeeds */
-static bool try_to_set_owner(struct task_struct *tsk, struct mm_struct *mm)
+static bool __try_to_set_owner(struct task_struct *tsk, struct mm_struct *mm)
{
bool ret = false;

@@ -456,12 +456,28 @@ static bool try_to_set_owner(struct task_struct *tsk, struct mm_struct *mm)
return ret;
}

+static bool try_to_set_owner(struct task_struct *g, struct mm_struct *mm)
+{
+ struct task_struct *t;
+
+ for_each_thread(g, t) {
+ struct mm_struct *t_mm = READ_ONCE(t->mm);
+ if (t_mm == mm) {
+ if (__try_to_set_owner(t, mm))
+ return true;
+ } else if (t_mm)
+ break;
+ }
+
+ return false;
+}
+
/*
* A task is exiting. If it owned this mm, find a new owner for the mm.
*/
void mm_update_next_owner(struct mm_struct *mm)
{
- struct task_struct *c, *g, *p = current;
+ struct task_struct *g, *p = current;

/*
* If the exiting or execing task is not the owner, it's
@@ -483,19 +499,17 @@ void mm_update_next_owner(struct mm_struct *mm)
/*
* Search in the children
*/
- list_for_each_entry(c, &p->children, sibling) {
- if (c->mm == mm && try_to_set_owner(c, mm))
+ list_for_each_entry(g, &p->children, sibling) {
+ if (try_to_set_owner(g, mm))
goto ret;
}
-
/*
* Search in the siblings
*/
- list_for_each_entry(c, &p->real_parent->children, sibling) {
- if (c->mm == mm && try_to_set_owner(c, mm))
+ list_for_each_entry(g, &p->real_parent->children, sibling) {
+ if (try_to_set_owner(g, mm))
goto ret;
}
-
/*
* Search through everything else, we should not get here often.
*/
@@ -504,14 +518,8 @@ void mm_update_next_owner(struct mm_struct *mm)
break;
if (g->flags & PF_KTHREAD)
continue;
- for_each_thread(g, c) {
- struct mm_struct *c_mm = READ_ONCE(c->mm);
- if (c_mm == mm) {
- if (try_to_set_owner(c, mm))
- goto ret;
- } else if (c_mm)
- break;
- }
+ if (try_to_set_owner(g, mm))
+ goto ret;
}
read_unlock(&tasklist_lock);
/*
--
2.25.1.362.g51ebf55