linux-kernel - Re: [PATCH 1/2] mm: Make alloc_contig

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Date:   Wed, 17 Feb 2021 14:53:37 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: Make alloc_contig_range handle free hugetlb pages

On 17.02.21 14:50, Michal Hocko wrote:
> On Wed 17-02-21 14:36:47, David Hildenbrand wrote:
>> On 17.02.21 14:30, Michal Hocko wrote:
>>> On Wed 17-02-21 11:08:15, Oscar Salvador wrote:
>>>> Free hugetlb pages are tricky to handle so as to no userspace application
>>>> notices disruption, we need to replace the current free hugepage with
>>>> a new one.
>>>>
>>>> In order to do that, a new function called alloc_and_dissolve_huge_page
>>>> is introduced.
>>>> This function will first try to get a new fresh hugetlb page, and if it
>>>> succeeds, it will dissolve the old one.
>>>>
>>>> With regard to the allocation, since we do not know whether the old page
>>>> was allocated on a specific node on request, the node the old page belongs
>>>> to will be tried first, and then we will fallback to all nodes containing
>>>> memory (N_MEMORY).
>>>
>>> I do not think fallback to a different zone is ok. If yes then this
>>> really requires a very good reasoning. alloc_contig_range is an
>>> optimistic allocation interface at best and it shouldn't break carefully
>>> node aware preallocation done by administrator.
>>
>> What does memory offlining do when migrating in-use hugetlbfs pages? Does it
>> always keep the node?
> 
> No it will break the node pool. The reasoning behind that is that
> offlining is an explicit request from the userspace and it is expected

userspace? in 99,9996% it's the hardware that triggers the unplug of a DIMM.

> 
>> I think keeping the node is the easiest/simplest approach for now.
>>
>>>
>>>> Note that gigantic hugetlb pages are fenced off since there is a cyclic
>>>> dependency between them and alloc_contig_range.
>>>
>>> Why do we need/want to do all this in the first place?
>>
>> cma and virtio-mem (especially on ZONE_MOVABLE) really want to handle
>> hugetlbfs pages.
> 
> 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.

-- 
Thanks,

David / dhildenb