Re: [PATCH v8 4/4] mm: hugetlb_vmemmap: add hugetlb_optimize_vmemmap sysctl

[Andrew Morton]

On Wed, 13 Apr 2022 22:47:48 +0800 Muchun Song <songmuchun@xxxxxxxxxxxxx> 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?