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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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