linux-kernel - Re: [PATCH 1/6] Revert "mm: mempolicy: Let vma_merge and vma_split handle vma->vm

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Message-ID: <CAHGf_=oLsK2bk_ym6EZfFD=uRpr33CL+1=nWmf4hrnCxUOFisQ@mail.gmail.com>
Date:	Wed, 30 May 2012 15:39:44 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...gle.com>, Dave Jones <davej@...hat.com>,
	Mel Gorman <mgorman@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	stable@...r.kernel.org, hughd@...gle.com,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/6] Revert "mm: mempolicy: Let vma_merge and vma_split
 handle vma->vm_policy linkages"

On Wed, May 30, 2012 at 3:17 PM, Christoph Lameter <cl@...ux.com> wrote:
> On Wed, 30 May 2012, kosaki.motohiro@...il.com wrote:
>
>> From: KOSAKI Motohiro <kosaki.motohiro@...il.com>
>>
>> commit 05f144a0d5 removed vma->vm_policy updates code and it is a purpose of
>> mbind_range(). Now, mbind_range() is virtually no-op. no-op function don't
>> makes any bugs, I agree. but maybe it is not right fix.
>
> I dont really understand the changelog. But to restore the policy_vma() is
> the right thing to do since there are potential multiple use cases where
> we want to apply a policy to a vma.
>
> Proposed new changelog:
>
> Commit 05f144a0d5 folded policy_vma() into mbind_range(). There are
> other use cases of policy_vma(*) though and so revert a piece of
> that commit in order to have a policy_vma() function again.
>
>> @@ -655,23 +676,9 @@ static int mbind_range(struct mm_struct *mm, unsigned long start,
>>                       if (err)
>>                               goto out;
>>               }
>> -
>> -             /*
>> -              * Apply policy to a single VMA. The reference counting of
>> -              * policy for vma_policy linkages has already been handled by
>> -              * vma_merge and split_vma as necessary. If this is a shared
>> -              * policy then ->set_policy will increment the reference count
>> -              * for an sp node.
>> -              */
>
> You are dropping the nice comments by Mel that explain the refcounting.

Because this is not strictly correct. 1) vma_merge() and split_vma() don't
care mempolicy refcount. They only dup and drop it. 2) This mpol_get() is
for vma attaching. This function don't need to care sp_node internal.
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