linux-kernel - Re: [RFC] mm/migrate: Add new migration reason MR

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Date:   Thu, 1 Feb 2018 13:58:36 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] mm/migrate: Add new migration reason MR_HUGETLB

On 02/01/2018 02:02 AM, Michal Hocko wrote:
> On Wed 31-01-18 12:12:17, Andrew Morton wrote:
>> On Wed, 31 Jan 2018 08:58:52 +0100 Michal Hocko <mhocko@...nel.org> wrote:
>>
>>> On Wed 31-01-18 07:55:05, Anshuman Khandual wrote:
>>>> On 01/30/2018 01:29 PM, Michal Hocko wrote:
>>>>> On Tue 30-01-18 08:37:14, Anshuman Khandual wrote:
>>>>>> alloc_contig_range() initiates compaction and eventual migration for
>>>>>> the purpose of either CMA or HugeTLB allocation. At present, reason
>>>>>> code remains the same MR_CMA for either of those cases. Lets add a
>>>>>> new reason code which will differentiate the purpose of migration
>>>>>> as HugeTLB allocation instead.
>>>>> Why do we need it?
>>>>
>>>> The same reason why we have MR_CMA (maybe some other ones as well) at
>>>> present, for reporting purpose through traces at the least. It just
>>>> seemed like same reason code is being used for two different purpose
>>>> of migration.
>>>
>>> But do we have any real user asking for this kind of information?
>>
>> It seems a reasonable cleanup: reusing MR_CMA for hugetlb just because
>> it happens to do the right thing is a bit hacky - the two things aren't
>> particularly related and a reader could be excused for feeling
>> confusion.
> 
> My bad! I thought this is a tracepoint thingy. But it seems to be only
> used as a migration reason for page_owner. Now it makes more sense.
>  
>> But the change seems incomplete:
>>
>>> +		if (migratetype == MIGRATE_CMA)
>>> +			migrate_reason = MR_CMA;
>>> +		else
>>> +			migrate_reason = MR_HUGETLB;
>>
>> If we're going to do this cleanup then shouldn't we go all the way and
>> add MIGRATE_HUGETLB?
> 
> Yes. We can expect more users of alloc_contig_range in future. Maybe we
> want to use MR_CONTIG_RANGE instead.

MR_CONTIG_RANGE can be a replacement for both MR_CMA and MR_HUGETLB.