linux-kernel - Re: [RFC PATCH 3/5] mm, hugetlb: do not rely on overcommit limit during migration

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Date:   Thu, 14 Dec 2017 12:57:54 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 3/5] mm, hugetlb: do not rely on overcommit limit
 during migration

On 12/13/2017 11:40 PM, Michal Hocko wrote:
> On Wed 13-12-17 15:35:33, Mike Kravetz wrote:
>> On 12/04/2017 06:01 AM, Michal Hocko wrote:
> [...]
>>> Before migration
>>> /sys/devices/system/node/node0/hugepages/hugepages-2048kB/free_hugepages:0
>>> /sys/devices/system/node/node0/hugepages/hugepages-2048kB/nr_hugepages:1
>>> /sys/devices/system/node/node0/hugepages/hugepages-2048kB/surplus_hugepages:0
>>> /sys/devices/system/node/node1/hugepages/hugepages-2048kB/free_hugepages:0
>>> /sys/devices/system/node/node1/hugepages/hugepages-2048kB/nr_hugepages:0
>>> /sys/devices/system/node/node1/hugepages/hugepages-2048kB/surplus_hugepages:0
>>>
>>> After
>>>
>>> /sys/devices/system/node/node0/hugepages/hugepages-2048kB/free_hugepages:0
>>> /sys/devices/system/node/node0/hugepages/hugepages-2048kB/nr_hugepages:0
>>> /sys/devices/system/node/node0/hugepages/hugepages-2048kB/surplus_hugepages:0
>>> /sys/devices/system/node/node1/hugepages/hugepages-2048kB/free_hugepages:0
>>> /sys/devices/system/node/node1/hugepages/hugepages-2048kB/nr_hugepages:1
>>> /sys/devices/system/node/node1/hugepages/hugepages-2048kB/surplus_hugepages:0
>>>
>>> with the previous implementation, both nodes would have nr_hugepages:1
>>> until the page is freed.
>>
>> With the previous implementation, the migration would have failed unless
>> nr_overcommit_hugepages was explicitly set.  Correct?
> 
> yes
> 
> [...]
> 
>> In the previous version of this patch, I asked about handling of 'free' huge
>> pages.  I did a little digging and IIUC, we do not attempt migration of
>> free huge pages.  The routine isolate_huge_page() has this check:
>>
>>         if (!page_huge_active(page) || !get_page_unless_zero(page)) {
>>                 ret = false;
>>                 goto unlock;
>>         }
>>
>> I believe one of your motivations for this effort was memory offlining.
>> So, this implies that a memory area can not be offlined if it contains
>> a free (not in use) huge page?
> 
> do_migrate_range will ignore this free huge page and then we will free
> it up in dissolve_free_huge_pages
> 
>> Just FYI and may be something we want to address later.
> 
> Maybe yes. The free pool might be reserved which would make
> dissolve_free_huge_pages to fail. Maybe we can be more clever and
> allocate a new huge page in that case.

Don't think we need to try and do anything more clever right now.  I was
just a little confused about the hot plug code.  Thanks for the explanation.

-- 
Mike Kravetz

>  
>> My other issues were addressed.
>>
>> Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>
> 
> Thanks!
>