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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
    >
    >
    >
    > --
    > To unsubscribe, send a message with 'unsubscribe linux-mm' in
    > the body to majordomo@kvack.org. For more info on Linux MM,
    > see: http://www.linux-mm.org/ .
    > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

    --
    Kind regards,
    Minchan Kim


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