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

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Message-ID: <YC0agxVWYRKGm5IO@dhcp22.suse.cz>
Date:   Wed, 17 Feb 2021 14:30:43 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        David Hildenbrand <david@...hat.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 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.

> 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?
-- 
Michal Hocko
SUSE Labs