> It's necessary to kill all threads that share an oom killed task's mm if
> the goal is to lead to future memory freeing.
>
> This patch reintroduces the code removed in 8c5cd6f3 (oom: oom_kill
> doesn't kill vfork parent (or child)) since it is obsoleted.
>
> It's now guaranteed that any task passed to oom_kill_task() does not
> share an mm with any thread that is unkillable. Thus, we're safe to
> issue a SIGKILL to any thread sharing the same mm.
correct.
>
> This is especially necessary to solve an mm->mmap_sem livelock issue
> whereas an oom killed thread must acquire the lock in the exit path while
> another thread is holding it in the page allocator while trying to
> allocate memory itself (and will preempt the oom killer since a task was
> already killed). Since tasks with pending fatal signals are now granted
> access to memory reserves, the thread holding the lock may quickly
> allocate and release the lock so that the oom killed task may exit.
I can't understand this sentence. mm sharing is happen when vfork, That
said, parent process is always sleeping. why do we need to worry that parent
process is holding mmap_sem?
Your change seems to don't change multi threading behavior. it only change
vfork() process behavior.
>
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
> ---
> v2: kill all threads in other thread groups before killing p to ensure
> it doesn't preemptively exit while still iterating through the
> tasklist and comparing unprotected mm pointers, as suggested by Oleg.
>
> mm/oom_kill.c | 20 ++++++++++++++++++++
> 1 files changed, 20 insertions(+), 0 deletions(-)
>
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -414,17 +414,37 @@ static void dump_header(struct task_struct *p, gfp_t gfp_mask, int order,
> #define K(x) ((x) << (PAGE_SHIFT-10))
> static int oom_kill_task(struct task_struct *p, struct mem_cgroup *mem)
> {
> + struct task_struct *q;
> + struct mm_struct *mm;
> +
> p = find_lock_task_mm(p);
> if (!p) {
> task_unlock(p);
> return 1;
> }
> +
> + /* mm cannot be safely dereferenced after task_unlock(p) */
> + mm = p->mm;
> +
> pr_err("Killed process %d (%s) total-vm:%lukB, anon-rss:%lukB, file-rss:%lukB\n",
> task_pid_nr(p), p->comm, K(p->mm->total_vm),
> K(get_mm_counter(p->mm, MM_ANONPAGES)),
> K(get_mm_counter(p->mm, MM_FILEPAGES)));
> task_unlock(p);
>
> + /*
> + * Kill all processes sharing p->mm in other thread groups, if any.
> + * They don't get access to memory reserves or a higher scheduler
> + * priority, though, to avoid depletion of all memory or task
> + * starvation. This prevents mm->mmap_sem livelock when an oom killed
> + * task cannot exit because it requires the semaphore and its contended
> + * by another thread trying to allocate memory itself. That thread will
> + * now get access to memory reserves since it has a pending fatal
> + * signal.
> + */
> + for_each_process(q)
> + if (q->mm == mm && !same_thread_group(q, p))
> + force_sig(SIGKILL, q);
This makes silent process kill when vfork() is used. right?
If so, it is wrong idea. instead, can you please write "which process was killed" log
on each process?
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