From: Michal Hocko <mhocko@xxxxxxxx> Subject: mm, oom: tighten task_will_free_mem() locking "mm, oom: fortify task_will_free_mem" has dropped task_lock around task_will_free_mem in oom_kill_process bacause it assumed that a potential race when the selected task exits will not be a problem as the oom_reaper will call exit_oom_victim. Tetsuo was objecting that nommu doesn't have oom_reaper so the race would be still possible. The code would be racy and lockup prone theoretically in other aspects without the oom reaper anyway so I didn't considered this a big deal. But it seems that further changes I am planning in this area will benefit from stable task->mm in this path as well. So let's drop find_lock_task_mm from task_will_free_mem and call it from under task_lock as we did previously. Just pull the task->mm != NULL check inside the function. Link: http://lkml.kernel.org/r/1467201562-6709-1-git-send-email-mhocko@xxxxxxxxxx Signed-off-by: Michal Hocko <mhocko@xxxxxxxx> Cc: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> Cc: Oleg Nesterov <oleg@xxxxxxxxxx> Cc: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx> Cc: David Rientjes <rientjes@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/oom_kill.c | 41 +++++++++++++++-------------------------- 1 file changed, 15 insertions(+), 26 deletions(-) diff -puN mm/oom_kill.c~mm-oom-fortify-task_will_free_mem-fix mm/oom_kill.c --- a/mm/oom_kill.c~mm-oom-fortify-task_will_free_mem-fix +++ a/mm/oom_kill.c @@ -757,45 +757,35 @@ static inline bool __task_will_free_mem( * Checks whether the given task is dying or exiting and likely to * release its address space. This means that all threads and processes * sharing the same mm have to be killed or exiting. + * Caller has to make sure that task->mm is stable (hold task_lock or + * it operates on the current). */ bool task_will_free_mem(struct task_struct *task) { - struct mm_struct *mm; + struct mm_struct *mm = task->mm; struct task_struct *p; bool ret; - if (!__task_will_free_mem(task)) - return false; - /* - * If the process has passed exit_mm we have to skip it because - * we have lost a link to other tasks sharing this mm, we do not - * have anything to reap and the task might then get stuck waiting - * for parent as zombie and we do not want it to hold TIF_MEMDIE + * Skip tasks without mm because it might have passed its exit_mm and + * exit_oom_victim. oom_reaper could have rescued that but do not rely + * on that for now. We can consider find_lock_task_mm in future. */ - p = find_lock_task_mm(task); - if (!p) + if (!mm) return false; - mm = p->mm; + if (!__task_will_free_mem(task)) + return false; /* * This task has already been drained by the oom reaper so there are * only small chances it will free some more */ - if (test_bit(MMF_OOM_REAPED, &mm->flags)) { - task_unlock(p); + if (test_bit(MMF_OOM_REAPED, &mm->flags)) return false; - } - if (atomic_read(&mm->mm_users) <= 1) { - task_unlock(p); + if (atomic_read(&mm->mm_users) <= 1) return true; - } - - /* pin the mm to not get freed and reused */ - atomic_inc(&mm->mm_count); - task_unlock(p); /* * This is really pessimistic but we do not have any reliable way @@ -812,7 +802,6 @@ bool task_will_free_mem(struct task_stru break; } rcu_read_unlock(); - mmdrop(mm); return ret; } @@ -838,12 +827,15 @@ void oom_kill_process(struct oom_control * If the task is already exiting, don't alarm the sysadmin or kill * its children or threads, just set TIF_MEMDIE so it can die quickly */ + task_lock(p); if (task_will_free_mem(p)) { mark_oom_victim(p); wake_oom_reaper(p); + task_unlock(p); put_task_struct(p); return; } + task_unlock(p); if (__ratelimit(&oom_rs)) dump_header(oc, p); @@ -1014,11 +1006,8 @@ bool out_of_memory(struct oom_control *o * If current has a pending SIGKILL or is exiting, then automatically * select it. The goal is to allow it to allocate so that it may * quickly exit and free its memory. - * - * But don't select if current has already released its mm and cleared - * TIF_MEMDIE flag at exit_mm(), otherwise an OOM livelock may occur. */ - if (current->mm && task_will_free_mem(current)) { + if (task_will_free_mem(current)) { mark_oom_victim(current); wake_oom_reaper(current); return true; _ -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
