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SubjectRe: [PATCH 3/5] oom: introduce find_lock_task_mm() to fix !mm false positives
On Mon, May 31, 2010 at 06:36:34PM +0900, KOSAKI Motohiro wrote:
> 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]
Reviewed-by: Minchan Kim <minchan.kim@gmail.com?

Good catch but I have a nitpick. :)

find_lock_task_mm isn't good name of the function, I think.
As you know, original goal of the function is to find sub-thread of p
which is alive(ie, doesn't release mm).

task_lock is important for user of the function but minor.

I suggest following as
/*
* If we find alive thread of process, it returns task_struct of sub thread.
* Notice. this function calls task_lock. So caller should call task_unlock.
*/
static struct task_struct *find_alive_subthread(struct task_struct *process)
{
...
}
I don't forced my suggesion if you suggest much good name.
Regardless of accepting my suggestion, looks good to me.

> ---
> 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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--
Kind regards,
Minchan Kim


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