Re: [PATCH] use find_lock_task_mm in memory cgroups oom

[Minchan Kim <minchan.kim@gmail.com>]

Hi, Kame.

On Tue, Jun 15, 2010 at 3:24 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@jp.fujitsu.com> wrote:
>
> based on  oom-introduce-find_lock_task_mm-to-fix-mm-false-positives.patch
> tested on mm-of-the-moment snapshot 2010-06-11-16-40.
>
> ==
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>
> When the OOM killer scans task, it check a task is under memcg or
> not when it's called via memcg's context.
>
> But, as Oleg pointed out, a thread group leader may have NULL ->mm
> and task_in_mem_cgroup() may do wrong decision. We have to use
> find_lock_task_mm() in memcg as generic OOM-Killer does.
>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

I have a trivial comment below.

> ---
>  include/linux/oom.h |    2 ++
>  mm/memcontrol.c     |   10 +++++++---
>  mm/oom_kill.c       |    8 ++++++--
>  3 files changed, 15 insertions(+), 5 deletions(-)
>
> Index: mmotm-2.6.35-0611/include/linux/oom.h
> ===================================================================
> --- mmotm-2.6.35-0611.orig/include/linux/oom.h
> +++ mmotm-2.6.35-0611/include/linux/oom.h
> @@ -45,6 +45,8 @@ static inline void oom_killer_enable(voi
>        oom_killer_disabled = false;
>  }
>
> +extern struct task_struct *find_lock_task_mm(struct task_struct *p);
> +
>  /* sysctls */
>  extern int sysctl_oom_dump_tasks;
>  extern int sysctl_oom_kill_allocating_task;
> Index: mmotm-2.6.35-0611/mm/memcontrol.c
> ===================================================================
> --- mmotm-2.6.35-0611.orig/mm/memcontrol.c
> +++ mmotm-2.6.35-0611/mm/memcontrol.c
> @@ -47,6 +47,7 @@
>  #include <linux/mm_inline.h>
>  #include <linux/page_cgroup.h>
>  #include <linux/cpu.h>
> +#include <linux/oom.h>
>  #include "internal.h"
>
>  #include <asm/uaccess.h>
> @@ -838,10 +839,13 @@ int task_in_mem_cgroup(struct task_struc
>  {
>        int ret;
>        struct mem_cgroup *curr = NULL;
> +       struct task_struct *p;
>
> -       task_lock(task);
> -       curr = try_get_mem_cgroup_from_mm(task->mm);
> -       task_unlock(task);
> +       p = find_lock_task_mm(task);
> +       if (!p)
> +               return 0;
> +       curr = try_get_mem_cgroup_from_mm(p->mm);
> +       task_unlock(p);
>        if (!curr)
>                return 0;
>        /*
> Index: mmotm-2.6.35-0611/mm/oom_kill.c
> ===================================================================
> --- mmotm-2.6.35-0611.orig/mm/oom_kill.c
> +++ mmotm-2.6.35-0611/mm/oom_kill.c
> @@ -81,13 +81,17 @@ static bool has_intersects_mems_allowed(
>  }
>  #endif /* CONFIG_NUMA */
>
> -/*
> +/**
> + * find_lock_task_mm - Checking a process which a task belongs to has valid mm
> + * and return a locked task which has a valid pointer to mm.
> + *

This comment should have been another patch.
BTW, below comment uses "subthread" word.
Personally it's easy to understand function's goal to me. :)

How about following as?
Checking a process which has any subthread with vaild mm
....


> + * @p: the task of a process to be checked.
>  * The process p may have detached its own ->mm while exiting or through
>  * use_mm(), but one or more of its subthreads may still have a valid
>  * pointer.  Return p, or any of its subthreads with a valid ->mm, with
>  * task_lock() held.
>  */
> -static struct task_struct *find_lock_task_mm(struct task_struct *p)
> +struct task_struct *find_lock_task_mm(struct task_struct *p)
>  {
>        struct task_struct *t = p;
>
>
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-- 
Kind regards,
Minchan Kim

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