Commit dd8e8f40 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds
Browse files

oom: introduce find_lock_task_mm() to fix !mm false positives 



Almost all ->mm == NULL 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 oom_badness() returns the
  task can be chosen if nothing else has been found yet.

  Change oom_badness() to return int, change it to return -1 if
  find_lock_task_mm() fails, and change select_bad_process() to
  check points >= 0.

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

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

	- oom_forkbomb_penalty() should use find_lock_task_mm() too,
	  and it also needs other changes to actually find the first
	  first-descendant children

This will be addressed later.

[kosaki.motohiro@jp.fujitsu.com: use in badness(), __oom_kill_task()]
Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
Signed-off-by: default avatarDavid Rientjes <rientjes@google.com>
Signed-off-by: default avatarKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent b5227940

mm/oom_kill.c

+43 −31
Original line number Diff line number Diff line
@@ -52,6 +52,20 @@ 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
@@ -74,8 +88,8 @@ 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;

	struct task_struct *c, *t;

	int oom_adj = p->signal->oom_adj;

	struct task_cputime task_time;

	unsigned long utime;
@@ -84,17 +98,14 @@ 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'
@@ -115,12 +126,17 @@ unsigned long badness(struct task_struct *p, unsigned long uptime) 
	 * child is eating the vast majority of memory, adding only half

	 * to the parents will make the child our kill candidate of choice.

	 */

	list_for_each_entry(child, &p->children, sibling) {

		task_lock(child);

		if (child->mm != mm && child->mm)

	t = p;

	do {

		list_for_each_entry(c, &t->children, sibling) {

			child = find_lock_task_mm(c);

			if (child) {

				if (child->mm != p->mm)

					points += child->mm->total_vm/2 + 1;

				task_unlock(child);

			}

		}

	} while_each_thread(p, t);



	/*

	 * CPU time is in tens of seconds and run time is in thousands
@@ -256,9 +272,6 @@ static struct task_struct *select_bad_process(unsigned long *ppoints, 
	for_each_process(p) {

		unsigned long points;



		/* skip tasks that 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;
@@ -385,14 +398,9 @@ static void __oom_kill_task(struct task_struct *p, int verbose) 
		return;

	}



	task_lock(p);

	if (!p->mm) {

		WARN_ON(1);

		printk(KERN_WARNING "tried to kill an mm-less task %d (%s)!\n",

			task_pid_nr(p), p->comm);

		task_unlock(p);

	p = find_lock_task_mm(p);

	if (!p)

		return;

	}



	if (verbose)

		printk(KERN_ERR "Killed process %d (%s) "
@@ -437,6 +445,7 @@ static int oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order, 
			    const char *message)

{

	struct task_struct *c;

	struct task_struct *t = p;



	if (printk_ratelimit())

		dump_header(p, gfp_mask, order, mem);
@@ -454,7 +463,8 @@ static int oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order, 
					message, task_pid_nr(p), p->comm, points);



	/* Try to kill a child first */

	list_for_each_entry(c, &p->children, sibling) {

	do {

		list_for_each_entry(c, &t->children, sibling) {

			if (c->mm == p->mm)

				continue;

			if (mem && !task_in_mem_cgroup(c, mem))
@@ -462,6 +472,8 @@ static int oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order, 
			if (!oom_kill_task(c))

				return 0;

		}

	} while_each_thread(p, t);



	return oom_kill_task(p);

}

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