Re: [PATCH v3 03/21] mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP

[Mike Kravetz <mike.kravetz@oracle.com>]

On 11/9/20 5:52 AM, Oscar Salvador wrote:
> On Sun, Nov 08, 2020 at 10:10:55PM +0800, Muchun Song wrote:
>> The purpose of introducing HUGETLB_PAGE_FREE_VMEMMAP is to configure
>> whether to enable the feature of freeing unused vmemmap associated
>> with HugeTLB pages. Now only support x86.
>>
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> ---
>>  arch/x86/mm/init_64.c |  2 +-
>>  fs/Kconfig            | 16 ++++++++++++++++
>>  mm/bootmem_info.c     |  3 +--
>>  3 files changed, 18 insertions(+), 3 deletions(-)
>>
>> 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..21b8d39a9715 100644
>> --- a/fs/Kconfig
>> +++ b/fs/Kconfig
>> @@ -245,6 +245,22 @@ config HUGETLBFS
>>  config HUGETLB_PAGE
>>  	def_bool HUGETLBFS
>>  
>> +config HUGETLB_PAGE_FREE_VMEMMAP
>> +	bool "Free unused vmemmap associated with HugeTLB pages"
>> +	default y
>> +	depends on X86
>> +	depends on HUGETLB_PAGE
>> +	depends on SPARSEMEM_VMEMMAP
>> +	depends on HAVE_BOOTMEM_INFO_NODE
>> +	help
>> +	  There are many struct page structures associated with each HugeTLB
>> +	  page. But we only use a few struct page structures. In this case,
>> +	  it wastes some memory. It is better to free the unused struct page
>> +	  structures to buddy system which can save some memory. For
>> +	  architectures that support it, say Y here.
>> +
>> +	  If unsure, say N.
> 
> I am not sure the above is useful for someone who needs to decide
> whether he needs/wants to enable this or not.
> I think the above fits better in a Documentation part.
> 
> I suck at this, but what about the following, or something along those
> lines? 
> 
> "
> When using SPARSEMEM_VMEMMAP, the system can save up some memory
> from pre-allocated HugeTLB pages when they are not used.
> 6 pages per 2MB HugeTLB page and 4095 per 1GB HugeTLB page.
> When the pages are going to be used or freed up, the vmemmap
> array representing that range needs to be remapped again and
> the pages we discarded earlier need to be rellocated again.
> Therefore, this is a trade-off between saving memory and
> increasing time in allocation/free path.
> "
> 
> It would be also great to point out that this might be a
> trade-off between saving up memory and increasing the cost
> of certain operations on allocation/free path.
> That is why I mentioned it there.

Yes, this is somewhat a trade-off.

As a config option, this is something that would likely be decided by
distros.  I almost hate to suggest this, but is it something that an
end user would want to decide?  Is this something that perhaps should
be a boot/kernel command line option?

-- 
Mike Kravetz