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Subject[PATCH 3/5] oom: introduce find_lock_task_mm() to fix !mm false positives
Date
From: Oleg Nesterov <oleg@redhat.com>
Subject: [PATCH 3/5] oom: introduce find_lock_task_mm() to fix !mm false positives

Almost all ->mm == NUL checks in oom_kill.c are wrong.

The current code assumes that the task without ->mm has already
released its memory and ignores the process. However this is not
necessarily true when this process is multithreaded, other live
sub-threads can use this ->mm.

- Remove the "if (!p->mm)" check in select_bad_process(), it is
just wrong.
- Add the new helper, find_lock_task_mm(), which finds the live
thread which uses the memory and takes task_lock() to pin ->mm

- change oom_badness() to use this helper instead of just checking
->mm != NULL.

- As David pointed out, select_bad_process() must never choose the
task without ->mm, but no matter what badness() returns the
task can be chosen if nothing else has been found yet.

Note! This patch is not enough, we need more changes.

- badness() was fixed, but oom_kill_task() still ignores
the task without ->mm

This will be addressed later.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> [rebase
latest -mm and remove some obsoleted description]
---
mm/oom_kill.c | 28 +++++++++++++++++-----------
1 files changed, 17 insertions(+), 11 deletions(-)
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index c87a6f4..162af2e 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -52,6 +52,19 @@ static int has_intersects_mems_allowed(struct task_struct *tsk)
return 0;
}

+static struct task_struct *find_lock_task_mm(struct task_struct *p)
+{
+ struct task_struct *t = p;
+ do {
+ task_lock(t);
+ if (likely(t->mm))
+ return t;
+ task_unlock(t);
+ } while_each_thread(p, t);
+
+ return NULL;
+}
+
/**
* badness - calculate a numeric value for how bad this task has been
* @p: task struct of which task we should calculate
@@ -74,7 +87,6 @@ static int has_intersects_mems_allowed(struct task_struct *tsk)
unsigned long badness(struct task_struct *p, unsigned long uptime)
{
unsigned long points, cpu_time, run_time;
- struct mm_struct *mm;
struct task_struct *child;
int oom_adj = p->signal->oom_adj;
struct task_cputime task_time;
@@ -84,17 +96,14 @@ unsigned long badness(struct task_struct *p, unsigned long uptime)
if (oom_adj == OOM_DISABLE)
return 0;

- task_lock(p);
- mm = p->mm;
- if (!mm) {
- task_unlock(p);
+ p = find_lock_task_mm(p);
+ if (!p)
return 0;
- }

/*
* The memory size of the process is the basis for the badness.
*/
- points = mm->total_vm;
+ points = p->mm->total_vm;

/*
* After this unlock we can no longer dereference local variable `mm'
@@ -117,7 +126,7 @@ unsigned long badness(struct task_struct *p, unsigned long uptime)
*/
list_for_each_entry(child, &p->children, sibling) {
task_lock(child);
- if (child->mm != mm && child->mm)
+ if (child->mm != p->mm && child->mm)
points += child->mm->total_vm/2 + 1;
task_unlock(child);
}
@@ -256,9 +265,6 @@ static struct task_struct *select_bad_process(unsigned long *ppoints,
for_each_process(p) {
unsigned long points;

- /* skip the tasks which have already released their mm. */
- if (!p->mm)
- continue;
/* skip the init task and kthreads */
if (is_global_init(p) || (p->flags & PF_KTHREAD))
continue;
--
1.6.5.2




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