Re: [PATCH v2 3/3] mm: hugetlb: introduce CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON

[Joao Martins <joao.m.martins@oracle.com>]

On 6/12/21 10:45 AM, Muchun Song wrote:
> When using HUGETLB_PAGE_FREE_VMEMMAP, the freeing unused vmemmap pages
> associated with each HugeTLB page is default off. Now the vmemmap is PMD
> mapped. So there is no side effect when this feature is enabled with no
> HugeTLB pages in the system. Someone may want to enable this feature in
> the compiler time instead of using boot command line. So add a config to
> make it default on when someone do not want to enable it via command line.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  Documentation/admin-guide/kernel-parameters.txt |  3 +++
>  fs/Kconfig                                      | 10 ++++++++++
>  mm/hugetlb_vmemmap.c                            |  6 ++++--
>  3 files changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index a01aadafee38..8eee439d943c 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1604,6 +1604,9 @@
>  			on:  enable the feature
>  			off: disable the feature
>  
> +			Built with CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON=y,
> +			the default is on.
> +
>  			This is not compatible with memory_hotplug.memmap_on_memory.
>  			If both parameters are enabled, hugetlb_free_vmemmap takes
>  			precedence over memory_hotplug.memmap_on_memory.
> diff --git a/fs/Kconfig b/fs/Kconfig
> index f40b5b98f7ba..e78bc5daf7b0 100644
> --- a/fs/Kconfig
> +++ b/fs/Kconfig
> @@ -245,6 +245,16 @@ config HUGETLB_PAGE_FREE_VMEMMAP
>  	depends on X86_64
>  	depends on SPARSEMEM_VMEMMAP
>  
Now that you have no longer have the directmap in basepages limitation, I suppose you no
longer need explicit arch support for HUGETLB_PAGE_FREE_VMEMMAP right?

If so, I suppose you might be able to remove the 'depends on X86_64' part and "gain"
ARM64, PPC, etc support.

	Joao