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Date:   Wed, 13 Apr 2022 12:10:51 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Muchun Song <songmuchun@bytedance.com>
Cc:     corbet@lwn.net, mike.kravetz@oracle.com, mcgrof@kernel.org,
        keescook@chromium.org, yzaikin@google.com, osalvador@suse.de,
        david@redhat.com, masahiroy@kernel.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        duanxiongchun@bytedance.com, smuchun@gmail.com
Subject: Re: [PATCH v8 4/4] mm: hugetlb_vmemmap: add
 hugetlb_optimize_vmemmap sysctl
Message-Id: <20220413121051.a363193c726451115c634a69@linux-foundation.org>
In-Reply-To: <20220413144748.84106-5-songmuchun@bytedance.com>
References: <20220413144748.84106-1-songmuchun@bytedance.com>
        <20220413144748.84106-5-songmuchun@bytedance.com>
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On Wed, 13 Apr 2022 22:47:48 +0800 Muchun Song <songmuchun@bytedance.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?