linux-kernel - Re: [PATCH -V9 04/15] hugetlb: use mmu_gather instead of a temporary linked list for accumulating pages

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Date:	Wed, 13 Jun 2012 16:59:23 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	linux-mm@...ck.org, kamezawa.hiroyu@...fujitsu.com,
	dhillf@...il.com, rientjes@...gle.com, akpm@...ux-foundation.org,
	hannes@...xchg.org, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org
Subject: Re: [PATCH -V9 04/15] hugetlb: use mmu_gather instead of a temporary
 linked list for accumulating pages

On Wed 13-06-12 15:57:23, Aneesh Kumar K.V wrote:
> From: "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
> 
> Use a mmu_gather instead of a temporary linked list for accumulating
> pages when we unmap a hugepage range

Sorry for coming up with the comment that late but you owe us an
explanation _why_ you are doing this.

I assume that this fixes a real problem when we take i_mmap_mutex
already up in 
unmap_mapping_range
  mutex_lock(&mapping->i_mmap_mutex);
  unmap_mapping_range_tree | unmap_mapping_range_list 
    unmap_mapping_range_vma
      zap_page_range_single
        unmap_single_vma
	  unmap_hugepage_range
	    mutex_lock(&vma->vm_file->f_mapping->i_mmap_mutex);

And that this should have been marked for stable as well (I haven't
checked when this has been introduced).

But then I do not see how this help when you still do this:
[...]
> diff --git a/mm/memory.c b/mm/memory.c
> index 1b7dc66..545e18a 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1326,8 +1326,11 @@ static void unmap_single_vma(struct mmu_gather *tlb,
>  			 * Since no pte has actually been setup, it is
>  			 * safe to do nothing in this case.
>  			 */
> -			if (vma->vm_file)
> -				unmap_hugepage_range(vma, start, end, NULL);
> +			if (vma->vm_file) {
> +				mutex_lock(&vma->vm_file->f_mapping->i_mmap_mutex);
> +				__unmap_hugepage_range(tlb, vma, start, end, NULL);
> +				mutex_unlock(&vma->vm_file->f_mapping->i_mmap_mutex);
> +			}
>  		} else
>  			unmap_page_range(tlb, vma, start, end, details);
>  	}

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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