linux-kernel - Re: [RFC] mm/mremap: Remove redundant checks inside vma

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Message-Id: <8bcc5908-7f0d-ba5c-a484-e0763f9b7664@linux.vnet.ibm.com>
Date:   Tue, 11 Jul 2017 16:38:46 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Vlastimil Babka <vbabka@...e.cz>, Michal Hocko <mhocko@...nel.org>
Cc:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, mike.kravetz@...cle.com
Subject: Re: [RFC] mm/mremap: Remove redundant checks inside vma_expandable()

On 07/11/2017 12:26 PM, Vlastimil Babka wrote:
> On 07/11/2017 08:50 AM, Michal Hocko wrote:
>> On Tue 11-07-17 08:26:40, Vlastimil Babka wrote:
>>> On 07/11/2017 08:03 AM, Michal Hocko wrote:
>>>>
>>>> Are you telling me that two if conditions cause more than a second
>>>> difference? That sounds suspicious.
>>>
>>> It's removing also a call to get_unmapped_area(), AFAICS. That means a
>>> vma search?
>>
>> Ohh, right. I have somehow missed that. Is this removal intentional?
> 
> I think it is: "Checking for availability of virtual address range at
> the end of the VMA for the incremental size is also reduntant at this
> point."
> 
>> The
>> changelog is silent about it.
> 
> It doesn't explain why it's redundant, indeed. Unfortunately, the commit
> f106af4e90ea ("fix checks for expand-in-place mremap") which added this,
> also doesn't explain why it's needed.

Its redundant because there are calls to get_unmapped_area() down the
line in the function whose failure will anyway fail the expansion of
the VMA.