Re: [PATCH v10 4/4] mm: hugetlb_vmemmap: add hugetlb_optimize_vmemmap sysctl

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

On 5/10/22 14:30, Mike Kravetz wrote:
> On 5/8/22 23:27, Muchun Song wrote:
>> diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
>> index 029fb7e26504..917112661b5c 100644
>> --- a/include/linux/memory_hotplug.h
>> +++ b/include/linux/memory_hotplug.h
>> @@ -351,4 +351,13 @@ void arch_remove_linear_mapping(u64 start, u64 size);
>>  extern bool mhp_supports_memmap_on_memory(unsigned long size);
>>  #endif /* CONFIG_MEMORY_HOTPLUG */
>>  
>> +#ifdef CONFIG_MHP_MEMMAP_ON_MEMORY
>> +bool mhp_memmap_on_memory(void);
>> +#else
>> +static inline bool mhp_memmap_on_memory(void)
>> +{
>> +	return false;
>> +}
>> +#endif
>> +
>>  #endif /* __LINUX_MEMORY_HOTPLUG_H */
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index 8605d7eb7f5c..86158eb9da70 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -1617,6 +1617,9 @@ static DECLARE_WORK(free_hpage_work, free_hpage_workfn);
>>  
>>  static inline void flush_free_hpage_work(struct hstate *h)
>>  {
>> +	if (!hugetlb_optimize_vmemmap_enabled())
>> +		return;
>> +
> 
> Hi Muchun,
> 
> In v9 I was suggesting that we may be able to eliminate the static_branch_inc/dec from the vmemmap free/alloc paths.  With this patch
> I believe hugetlb_optimize_vmemmap_enabled() is really checking
> 'has hugetlb vmemmap optimization been enabled' OR 'are there still vmemmap
> optimized hugetlb pages in the system'.  That may be confusing.
> 

Sorry, I forgot about the use of hugetlb_optimize_vmemmap_enabled in
page_fixed_fake_head.  We need to know if there are any vmemmap optimized
hugetlb pages in the system in this performance sensitive path.  So,
static_branch_inc/dec is indeed a good idea.

Please disregard my attempt below at removing static_branch_inc/dec.

I still find the name hugetlb_optimize_vmemmap_enabled a bit confusing as
it tests two conditions (enabled and pages in use).

You have already 'open coded' just the check for enabled in the routine
hugetlb_vmemmap_free with:

	READ_ONCE(vmemmap_optimize_mode) == VMEMMAP_OPTIMIZE_OFF

How about having hugetlb_optimize_vmemmap_enabled() just check
vmemmap_optimize_mode in a manner like above?  Then rename
hugetlb_optimize_vmemmap_enabled to something like:
hugetlb_optimized_vmemmap_possible().  Sorry, I can think if a great name.
-- 
Mike Kravetz