Re: can't oom-kill zap the victim's memory?

[Linus Torvalds]

On Sat, Sep 19, 2015 at 8:03 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> +
> +static void oom_unmap_func(struct work_struct *work)
> +{
> +       struct mm_struct *mm = xchg(&oom_unmap_mm, NULL);
> +
> +       if (!atomic_inc_not_zero(&mm->mm_users))
> +               return;
> +
> +       // If this is not safe we can do use_mm() + unuse_mm()
> +       down_read(&mm->mmap_sem);

I don't think this is safe.

What makes you sure that we might not deadlock on the mmap_sem here?
For all we know, the process that is going out of memory is in the
middle of a mmap(), and already holds the mmap_sem for writing. No?

So at the very least that needs to be a trylock, I think. And I'm not
sure zap_page_range() is ok with the mmap_sem only held for reading.
Normally our rule is that you can *populate* the page tables
concurrently, but you can't tear the down.

                Linus
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/