Re: [PATCH v2] hugetlb: allow to free gigantic pages regardless of the configuration

[Vlastimil Babka <vbabka@suse.cz>]

On 2/13/19 8:30 PM, Dave Hansen wrote:
>> -#if (defined(CONFIG_MEMORY_ISOLATION) && defined(CONFIG_COMPACTION)) || defined(CONFIG_CMA)
>> +#ifdef CONFIG_COMPACTION_CORE
>>  static __init int gigantic_pages_init(void)
>>  {
>>  	/* With compaction or CMA we can allocate gigantic pages at runtime */
>> diff --git a/fs/Kconfig b/fs/Kconfig
>> index ac474a61be37..8fecd3ea5563 100644
>> --- a/fs/Kconfig
>> +++ b/fs/Kconfig
>> @@ -207,8 +207,9 @@ config HUGETLB_PAGE
>>  config MEMFD_CREATE
>>  	def_bool TMPFS || HUGETLBFS
>>  
>> -config ARCH_HAS_GIGANTIC_PAGE
>> +config COMPACTION_CORE
>>  	bool
>> +	default y if (MEMORY_ISOLATION && MIGRATION) || CMA
> 
> This takes a hard dependency (#if) and turns it into a Kconfig *default*
> that can be overridden.  That seems like trouble.
> 
> Shouldn't it be:
> 
> config COMPACTION_CORE
> 	def_bool y
> 	depends on (MEMORY_ISOLATION && MIGRATION) || CMA

Agreed. Also I noticed that it now depends on MIGRATION instead of
COMPACTION. That intention is correct IMHO, but will fail to
compile/link when both COMPACTION and CMA are disabled, and would need
more changes in mm/internal.h and mm/compaction.c (possibly just
replacing CMA in all "if defined CONFIG_COMPACTION || defined
CONFIG_CMA" instances with COMPACTION_CORE, but there might be more
problems, wanna try? :)

Also, I realized that COMPACTION_CORE is a wrong name, sorry about that.
What the config really provides is alloc_contig_range(), so it should be
named either CONFIG_CMA_CORE (as it provides contiguous memory
allocation, but not the related reservation and accounting), or
something like CONFIG_CONTIG_ALLOC. I would also move it from fs/Kconfig
to mm/Kconfig.

Thanks!