linux-kernel - Re: [PATCH v8 4/4] mm: hugetlb_vmemmap: add hugetlb_optimize

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Date:   Wed, 13 Apr 2022 12:10:51 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     corbet@....net, mike.kravetz@...cle.com, mcgrof@...nel.org,
        keescook@...omium.org, yzaikin@...gle.com, osalvador@...e.de,
        david@...hat.com, masahiroy@...nel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        duanxiongchun@...edance.com, smuchun@...il.com
Subject: Re: [PATCH v8 4/4] mm: hugetlb_vmemmap: add
 hugetlb_optimize_vmemmap sysctl

On Wed, 13 Apr 2022 22:47:48 +0800 Muchun Song <songmuchun@...edance.com> wrote:

> We must add hugetlb_free_vmemmap=on (or "off") to the boot cmdline and
> reboot the server to enable or disable the feature of optimizing vmemmap
> pages associated with HugeTLB pages.  However, rebooting usually takes a
> long time.  So add a sysctl to enable or disable the feature at runtime
> without rebooting.

Do we really need this feature?  Really?  What's the use case and what
is the end-user value?

Presumably CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP worsens things for some
setups/workloads?  Please tell us much more about that.  What is the
magnitude of the deoptimization?