linux-kernel - Re: [PATCH v4 1/4] lazy tlb: introduce lazy mm refcount helper functions

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Message-Id: <20210607164934.d453adcc42473e84beb25db3@linux-foundation.org>
Date:   Mon, 7 Jun 2021 16:49:34 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Randy Dunlap <rdunlap@...radead.org>, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-mm@...ck.org, Anton Blanchard <anton@...abs.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v4 1/4] lazy tlb: introduce lazy mm refcount helper
 functions

On Sat,  5 Jun 2021 11:42:13 +1000 Nicholas Piggin <npiggin@...il.com> wrote:

> Add explicit _lazy_tlb annotated functions for lazy mm refcounting.
> This makes lazy mm references more obvious, and allows explicit
> refcounting to be removed if it is not used.
> 
> ...
>
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -1314,14 +1314,14 @@ void kthread_use_mm(struct mm_struct *mm)
>  	WARN_ON_ONCE(!(tsk->flags & PF_KTHREAD));
>  	WARN_ON_ONCE(tsk->mm);
>  
> +	mmgrab(mm);
> +
>  	task_lock(tsk);
>  	/* Hold off tlb flush IPIs while switching mm's */
>  	local_irq_disable();
>  	active_mm = tsk->active_mm;
> -	if (active_mm != mm) {
> -		mmgrab(mm);
> +	if (active_mm != mm)
>  		tsk->active_mm = mm;
> -	}

Looks like a functional change.  What's happening here?