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

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Date:   Tue, 11 Jul 2017 08:56:04 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     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 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.