linux-kernel - Re: [RFC PATCH 0/3] hugetlb: add demote/split page functionality

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Message-ID: <YEjyS+xyeNlMcW/l@dhcp22.suse.cz>
Date:   Wed, 10 Mar 2021 17:23:09 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>, Zi Yan <ziy@...dia.com>,
        David Rientjes <rientjes@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 0/3] hugetlb: add demote/split page functionality

On Mon 08-03-21 16:18:52, Mike Kravetz wrote:
[...]
> Converting larger to smaller hugetlb pages can be accomplished today by
> first freeing the larger page to the buddy allocator and then allocating
> the smaller pages.  However, there are two issues with this approach:
> 1) This process can take quite some time, especially if allocation of
>    the smaller pages is not immediate and requires migration/compaction.
> 2) There is no guarantee that the total size of smaller pages allocated
>    will match the size of the larger page which was freed.  This is
>    because the area freed by the larger page could quickly be
>    fragmented.

I will likely not surprise to show some level of reservation. While your
concerns about reconfiguration by existing interfaces are quite real is
this really a problem in practice? How often do you need such a
reconfiguration?

Is this all really worth the additional code to something as tricky as
hugetlb code base?

>  include/linux/hugetlb.h |   8 ++
>  mm/hugetlb.c            | 199 +++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 204 insertions(+), 3 deletions(-)
> 
> -- 
> 2.29.2
> 

-- 
Michal Hocko
SUSE Labs