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

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Date:   Wed, 17 Feb 2021 14:36:47 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...e.com>, Oscar Salvador <osalvador@...e.de>
Cc:     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 14:30, 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?

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.

-- 
Thanks,

David / dhildenb