linux-kernel - Re: [PATCH v3 0/2] Make alloc_contig

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Message-ID: <8c230cb6-9c29-eaec-c548-46199611b1e6@redhat.com>
Date:   Mon, 1 Mar 2021 13:59:23 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/2] Make alloc_contig_range handle Hugetlb pages

On 01.03.21 13:57, Oscar Salvador wrote:
> On Mon, Mar 01, 2021 at 01:43:00PM +0100, David Hildenbrand wrote:
>> Same experiment with ZONE_MOVABLE:
>>
>> a) Free huge pages: all memory can get unplugged again.
>>
>> b) Allocated/populated but idle huge pages: all memory can get unplugged
>> again.
>>
>> c) Allocated/populated but all 512 huge pages are read/written in a loop:
>> all memory can get unplugged again, but I get a single
>>
>> [  121.192345] alloc_contig_range: [180000, 188000) PFNs busy
>>
>> Most probably because it happened to try migrating a huge page while it was
>> busy. As virtio-mem retries on ZONE_MOVABLE a couple of times, it can deal
>> with this temporary failure.
>>
>>
>>
>> Last but not least, I did something extreme:
>>
>> ]# cat /proc/meminfo
>> MemTotal:        5061568 kB
>> MemFree:          186560 kB
>> MemAvailable:     354524 kB
>> ...
>> HugePages_Total:    2048
>> HugePages_Free:     2048
>> HugePages_Rsvd:        0
>> HugePages_Surp:        0
>>
>>
>> Triggering unplug would require to dissolve+alloc - which now fails when
>> trying to allocate an additional ~512 huge pages (1G).
>>
>>
>> As expected, I can properly see memory unplug not fully succeeding. + I get
>> a fairly continuous stream of
>>
>> [  226.611584] alloc_contig_range: [19f400, 19f800) PFNs busy
>> ...
>>
>> But more importantly, the hugepage count remains stable, as configured by
>> the admin (me):
>>
>> HugePages_Total:    2048
>> HugePages_Free:     2048
>> HugePages_Rsvd:        0
>> HugePages_Surp:        0
> 
> Thanks for giving it a spin David, that is highly appreciated ;-)!
> 
> I will add above information in next's version changelog if you do not mind,
> so the before-and-after can be seen clearly.
> 
> I shall send v4 in the course of the next few days.
> 

I'll have some review feedback on error handling that might be improved, 
I'll share that shortly.

-- 
Thanks,

David / dhildenb