linux-kernel - Re: [PATCH] mm: vmalloc: Use the vmap_area_lock to protect ne_fit_preload

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Date:   Fri, 4 Oct 2019 17:37:28 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Daniel Wagner <dwagner@...e.de>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-rt-users@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Uladzislau Rezki <urezki@...il.com>
Subject: Re: [PATCH] mm: vmalloc: Use the vmap_area_lock to protect
 ne_fit_preload_node

If you post something that is related to PREEMPT_RT please keep tglx and
me in Cc.

On 2019-10-03 11:09:06 [+0200], Daniel Wagner wrote:
> Replace preempt_enable() and preempt_disable() with the vmap_area_lock
> spin_lock instead. Calling spin_lock() with preempt disabled is
> illegal for -rt. Furthermore, enabling preemption inside the
> spin_lock() doesn't really make sense.

This spin_lock will cause all CPUs to block on it while the
preempt_disable() does not have this limitation.
I added a migrate_disable() in my -next tree. Looking at it again, I
have reasonable doubt that this preempt_disable() is needed.

> Fixes: 82dd23e84be3 ("mm/vmalloc.c: preload a CPU with one object for
> split purpose")
> Cc: Uladzislau Rezki (Sony) <urezki@...il.com>
> Signed-off-by: Daniel Wagner <dwagner@...e.de>
> ---
>  mm/vmalloc.c | 9 +++------
>  1 file changed, 3 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 08c134aa7ff3..0d1175673583 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1091,11 +1091,11 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
>  	 * Even if it fails we do not really care about that. Just proceed
>  	 * as it is. "overflow" path will refill the cache we allocate from.
>  	 */
> -	preempt_disable();
> +	spin_lock(&vmap_area_lock);
>  	if (!__this_cpu_read(ne_fit_preload_node)) {
> -		preempt_enable();
> +		spin_unlock(&vmap_area_lock);
>  		pva = kmem_cache_alloc_node(vmap_area_cachep, GFP_KERNEL, node);
> -		preempt_disable();
> +		spin_lock(&vmap_area_lock);
>  
>  		if (__this_cpu_cmpxchg(ne_fit_preload_node, NULL, pva)) {
>  			if (pva)
> @@ -1103,9 +1103,6 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
>  		}
>  	}
>  
> -	spin_lock(&vmap_area_lock);
> -	preempt_enable();
> -
>  	/*
>  	 * If an allocation fails, the "vend" address is
>  	 * returned. Therefore trigger the overflow path.
> -- 

Sebastian