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To:     Michal Hocko <mhocko@suse.com>
Cc:     Oscar Salvador <osalvador@suse.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Subject: Re: [PATCH 1/2] mm: Make alloc_contig_range handle free hugetlb pages
Message-ID: <5d70b340-2db0-ef1f-1564-e5d39354c11c@redhat.com>
Date:   Wed, 17 Feb 2021 15:08:04 +0100
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On 17.02.21 14:59, Michal Hocko wrote:
> On Wed 17-02-21 14:53:37, David Hildenbrand wrote:
>> On 17.02.21 14:50, Michal Hocko wrote:
> [...]
>>> Do we have any real life examples? Or does this fall more into, let's
>>> optimize an existing implementation category.
>>>
>>
>> It's a big TODO item I have on my list and I am happy that Oscar is looking
>> into it. So yes, I noticed it while working on virtio-mem. It's real.
> 
> Do not take me wrong, I am not opposing to the functionality. I am
> asking for the specific usecase.

Makes sense, and a proper motivation should be included in the 
patches/cover letter. So here comes a quick-n-dirty example:


Start a VM with 4G. Hotplug 1G via virtio-mem and online it to 
ZONE_MOVABLE. Allocate 512 huge pages.

[root@localhost ~]# cat /proc/meminfo
MemTotal:        5061512 kB
MemFree:         3319396 kB
MemAvailable:    3457144 kB
...
HugePages_Total:     512
HugePages_Free:      512
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB


The huge pages get partially allocate from ZONE_MOVABLE. Try unplugging 
1G via virtio-mem (remember, all ZONE_MOVABLE). Inside the guest:

[  180.058992] alloc_contig_range: [1b8000, 1c0000) PFNs busy
[  180.060531] alloc_contig_range: [1b8000, 1c0000) PFNs busy
[  180.061972] alloc_contig_range: [1b8000, 1c0000) PFNs busy
[  180.063413] alloc_contig_range: [1b8000, 1c0000) PFNs busy
[  180.064838] alloc_contig_range: [1b8000, 1c0000) PFNs busy
[  180.065848] alloc_contig_range: [1bfc00, 1c0000) PFNs busy
[  180.066794] alloc_contig_range: [1bfc00, 1c0000) PFNs busy
[  180.067738] alloc_contig_range: [1bfc00, 1c0000) PFNs busy
[  180.068669] alloc_contig_range: [1bfc00, 1c0000) PFNs busy
[  180.069598] alloc_contig_range: [1bfc00, 1c0000) PFNs busy


I succeed in unplugging 540MB - 484 MB remain blocked by huge pages 
("which did not end up there by pure luck"). These pages are movable 
(and even free!) and can easily be reallocated.

-- 
Thanks,

David / dhildenb