linux-kernel - Re: [PATCH 4/5] teach smaps_pte

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Date:	Thu, 10 Feb 2011 19:08:02 +0100
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Michael J Wolf <mjwolf@...ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH 4/5] teach smaps_pte_range() about THP pmds

On Wed, Feb 09, 2011 at 11:54:11AM -0800, Dave Hansen wrote:
> @@ -385,8 +387,16 @@ static int smaps_pte_range(pmd_t *pmd, u
>  	pte_t *pte;
>  	spinlock_t *ptl;
>  
> -	split_huge_page_pmd(walk->mm, pmd);
> -
> +	if (pmd_trans_huge(*pmd)) {
> +		if (pmd_trans_splitting(*pmd)) {
> +			spin_unlock(&walk->mm->page_table_lock);
> +			wait_split_huge_page(vma->anon_vma, pmd);
> +			spin_lock(&walk->mm->page_table_lock);

the locking looks wrong, who is taking the &walk->mm->page_table_lock,
and isn't this going to deadlock on the pte_offset_map_lock for
NR_CPUS < 4, and where is it released? This spin_lock don't seem
necessary to me.

The right locking would be:

 spin_lock(&walk->mm->page_table_lock);
 if (pmd_trans_huge(*pmd)) {
   if (pmd_trans_splitting(*pmd)) {
    spin_unlock(&walk->mm->page_table_lock);
    wait_split_huge_page(vma->anon_vma, pmd);
   } else {
    smaps_pte_entry(*(pte_t *)pmd, addr, HPAGE_SIZE, walk);
    spin_unlock(&walk->mm->page_table_lock);
    return 0;
  }

I think it worked because you never run into a pmd_trans_splitting pmd
yet, and you were running smaps_pte_entry lockless which could race
against split_huge_page (but it normally doesn't).

> +		} else {
> +			smaps_pte_entry(*(pte_t *)pmd, addr, HPAGE_SIZE, walk);
> +			return 0;
> +		}
> +	}
>  	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
>  	for (; addr != end; pte++, addr += PAGE_SIZE)
>  		smaps_pte_entry(*pte, addr, PAGE_SIZE, walk);
> _
> 
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