linux-kernel - Re: [patch for-3.7 v2] mm, mempolicy: avoid taking mutex inside spinlock when reading numa

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Date:	Wed, 17 Oct 2012 21:35:53 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	bhutchings@...arflare.com,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [patch for-3.7 v2] mm, mempolicy: avoid taking mutex inside
 spinlock when reading numa_maps

On Thu, 18 Oct 2012, Kamezawa Hiroyuki wrote:

> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 14df880..d92e868 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -94,6 +94,11 @@ static void vma_stop(struct proc_maps_private *priv, struct
> vm_area_struct *vma)
>  {
>  	if (vma && vma != priv->tail_vma) {
>  		struct mm_struct *mm = vma->vm_mm;
> +#ifdef CONFIG_NUMA
> +		task_lock(priv->task);
> +		__mpol_put(priv->task->mempolicy);
> +		task_unlock(priv->task);
> +#endif
>  		up_read(&mm->mmap_sem);
>  		mmput(mm);
>  	}
> @@ -130,6 +135,16 @@ static void *m_start(struct seq_file *m, loff_t *pos)
>  		return mm;
>  	down_read(&mm->mmap_sem);
>  +	/*
> +	 * task->mempolicy can be freed even if mmap_sem is down (see
> kernel/exit.c)
> +	 * We grab refcount for stable access.
> +	 * repleacement of task->mmpolicy is guarded by mmap_sem.
> +	 */
> +#ifdef CONFIG_NUMA
> +	task_lock(priv->task);
> +	mpol_get(priv->task->mempolicy);
> +	task_unlock(priv->task);
> +#endif
>  	tail_vma = get_gate_vma(priv->task->mm);
>  	priv->tail_vma = tail_vma;
>  @@ -161,6 +176,11 @@ out:
>   	/* End of vmas has been reached */
>  	m->version = (tail_vma != NULL)? 0: -1UL;
> +#ifdef CONFIG_NUMA
> +	task_lock(priv->task);
> +	__mpol_put(priv->task->mempolicy);
> +	task_unlock(priv->task);
> +#endif
>  	up_read(&mm->mmap_sem);
>  	mmput(mm);
>  	return tail_vma;

Yes, I must admit that this is better than my version and it looks like 
all the ->show() functions that use these start, next, stop functions 
don't take task_lock() and this would generally be useful: we already hold 
current->mm->mmap_sem so there is little harm in holding 
task_lock(current) when reading these files as long as we're not touching 
the fastpath.

These routines seem like it would nicely be added to mempolicy.h since we 
depend on CONFIG_NUMA there already.

Please fix up the mess I made in show_numa_map() in 32f8516a8c73 ("mm, 
mempolicy: fix printing stack contents in numa_maps") by simply removing 
the task_lock() and task_unlock() as part of your patch.

Thanks Kame!
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