Re: [PATCH] mm: Check for SIGKILL inside dup_mmap() loop.

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 7 Apr 2018 19:38:28 +0900 Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:

> >From 31c863e57a4ab7dfb491b2860fe3653e1e8f593b Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Date: Sat, 7 Apr 2018 19:29:30 +0900
> Subject: [PATCH] mm: Check for SIGKILL inside dup_mmap() loop.
> 
> As a theoretical problem, an mm_struct with 60000+ vmas can loop with
> potentially allocating memory, with mm->mmap_sem held for write by current
> thread. This is bad if current thread was selected as an OOM victim, for
> current thread will continue allocations using memory reserves while OOM
> reaper is unable to reclaim memory.
> 
> As an actually observable problem, it is not difficult to make OOM reaper
> unable to reclaim memory if the OOM victim is blocked at
> i_mmap_lock_write() in this loop. Unfortunately, since nobody can explain
> whether it is safe to use killable wait there, let's check for SIGKILL
> before trying to allocate memory. Even without an OOM event, there is no
> point with continuing the loop from the beginning if current thread is
> killed.
> 
> ...
>
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -441,6 +441,10 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>  			continue;
>  		}
>  		charge = 0;
> +		if (fatal_signal_pending(current)) {
> +			retval = -EINTR;
> +			goto out;
> +		}
>  		if (mpnt->vm_flags & VM_ACCOUNT) {
>  			unsigned long len = vma_pages(mpnt);

Seems sane.  Has this been runtime tested?

I would like to see a comment here explaining why we're testing for
this at this particualr place.