Subject: Re: [RFC] mm/mremap: Remove redundant checks inside vma_expandable()
To: Michal Hocko <mhocko@kernel.org>
Cc: Anshuman Khandual <khandual@linux.vnet.ibm.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
        mike.kravetz@oracle.com
References: <20170710111059.30795-1-khandual@linux.vnet.ibm.com>
 <20170710134917.GB19645@dhcp22.suse.cz>
 <d6f9ec12-4518-8f97-eca9-6592808b839d@linux.vnet.ibm.com>
 <20170711060354.GA24852@dhcp22.suse.cz>
 <4c182da0-6c84-df67-b173-6960fac0544a@suse.cz>
 <20170711065030.GE24852@dhcp22.suse.cz>
From: Vlastimil Babka <vbabka@suse.cz>
Message-ID: <337a8a4c-1f27-7371-409d-6a9f181b3871@suse.cz>
Date: Tue, 11 Jul 2017 08:56:04 +0200
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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.