Re: [PATCH v13 02/12] mm: hugetlb: introduce a new config HUGETLB_PAGE_FREE_VMEMMAP

[Randy Dunlap <rdunlap@infradead.org>]

On 1/24/21 3:58 PM, David Rientjes wrote:
> On Sun, 17 Jan 2021, Muchun Song wrote:
> 
>> The HUGETLB_PAGE_FREE_VMEMMAP option is used to enable the freeing
>> of unnecessary vmemmap associated with HugeTLB pages. The config
>> option is introduced early so that supporting code can be written
>> to depend on the option. The initial version of the code only
>> provides support for x86-64.
>>
>> Like other code which frees vmemmap, this config option depends on
>> HAVE_BOOTMEM_INFO_NODE. The routine register_page_bootmem_info() is
>> used to register bootmem info. Therefore, make sure
>> register_page_bootmem_info is enabled if HUGETLB_PAGE_FREE_VMEMMAP
>> is defined.
>>
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> Reviewed-by: Oscar Salvador <osalvador@suse.de>
>> Acked-by: Mike Kravetz <mike.kravetz@oracle.com>
>> ---
>>  arch/x86/mm/init_64.c |  2 +-
>>  fs/Kconfig            | 18 ++++++++++++++++++
>>  2 files changed, 19 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
>> index 0a45f062826e..0435bee2e172 100644
>> --- a/arch/x86/mm/init_64.c
>> +++ b/arch/x86/mm/init_64.c
>> @@ -1225,7 +1225,7 @@ static struct kcore_list kcore_vsyscall;
>>  
>>  static void __init register_page_bootmem_info(void)
>>  {
>> -#ifdef CONFIG_NUMA
>> +#if defined(CONFIG_NUMA) || defined(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP)
>>  	int i;
>>  
>>  	for_each_online_node(i)
>> diff --git a/fs/Kconfig b/fs/Kconfig
>> index 976e8b9033c4..e7c4c2a79311 100644
>> --- a/fs/Kconfig
>> +++ b/fs/Kconfig
>> @@ -245,6 +245,24 @@ config HUGETLBFS
>>  config HUGETLB_PAGE
>>  	def_bool HUGETLBFS
>>  
>> +config HUGETLB_PAGE_FREE_VMEMMAP
>> +	def_bool HUGETLB_PAGE
> 
> I'm not sure I understand the rationale for providing this help text if 
> this is def_bool depending on CONFIG_HUGETLB_PAGE.  Are you intending that 
> this is actually configurable and we want to provide guidance to the admin 
> on when to disable it (which it currently doesn't)?  If not, why have the 
> help text?

It's good for the (non-user) Kconfig symbol's meaning to be documented somewhere,
preferably such that one does not have to go digging thru git commit logs
to find it.

>> +	depends on X86_64
>> +	depends on SPARSEMEM_VMEMMAP
>> +	depends on HAVE_BOOTMEM_INFO_NODE
>> +	help
>> +	  The option HUGETLB_PAGE_FREE_VMEMMAP allows for the freeing of
>> +	  some vmemmap pages associated with pre-allocated HugeTLB pages.
>> +	  For example, on X86_64 6 vmemmap pages of size 4KB each can be
>> +	  saved for each 2MB HugeTLB page.  4094 vmemmap pages of size 4KB
>> +	  each can be saved for each 1GB HugeTLB page.
>> +
>> +	  When a HugeTLB page is allocated or freed, the vmemmap array
>> +	  representing the range associated with the page will need to be
>> +	  remapped.  When a page is allocated, vmemmap pages are freed
>> +	  after remapping.  When a page is freed, previously discarded
>> +	  vmemmap pages must be allocated before remapping.
>> +
>>  config MEMFD_CREATE
>>  	def_bool TMPFS || HUGETLBFS
>>  
> 


-- 
~Randy