Re: [RFC 2/3] mm: add STATIC_PMD_ZERO_PAGE config option

[David Hildenbrand <david@redhat.com>]

On 27.05.25 20:00, Pankaj Raghav (Samsung) wrote:
> On Tue, May 27, 2025 at 09:37:50AM -0700, Dave Hansen wrote:
>>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>>> index 055204dc211d..96f99b4f96ea 100644
>>> --- a/arch/x86/Kconfig
>>> +++ b/arch/x86/Kconfig
>>> @@ -152,6 +152,7 @@ config X86
>>>   	select ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP	if X86_64
>>>   	select ARCH_WANT_HUGETLB_VMEMMAP_PREINIT if X86_64
>>>   	select ARCH_WANTS_THP_SWAP		if X86_64
>>> +	select ARCH_WANTS_STATIC_PMD_ZERO_PAGE if X86_64
>>
>> I don't think this should be the default. There are lots of little
>> x86_64 VMs sitting around and 2MB might be significant to them.
> 
> This is the feedback I wanted. I will make it optional.
> 
>>> +config ARCH_WANTS_STATIC_PMD_ZERO_PAGE
>>> +	bool
>>> +
>>> +config STATIC_PMD_ZERO_PAGE
>>> +	def_bool y
>>> +	depends on ARCH_WANTS_STATIC_PMD_ZERO_PAGE
>>> +	help
>>> +	  Typically huge_zero_folio, which is a PMD page of zeroes, is allocated
>>> +	  on demand and deallocated when not in use. This option will always
>>> +	  allocate huge_zero_folio for zeroing and it is never deallocated.
>>> +	  Not suitable for memory constrained systems.
>>
>> "Static" seems like a weird term to use for this. I was really expecting
>> to see a 2MB object that gets allocated in .bss or something rather than
>> a dynamically allocated page that's just never freed.
> 
> My first proposal was along those lines[0] (sorry I messed up version
> while sending the patches). David Hilderbrand suggested to leverage the
> infrastructure we already have in huge_memory.

Sorry, maybe I was not 100% clear.

We could either

a) Allocate it statically in bss and reuse it for huge_memory purposes

(static vs. dynamic is a good fit)

b) Allocate it during early boot and never free it.

Assuming we allocate it from memblock, it's almost static ... :)


I would not allocate it at runtime later when requested. Then, "static" 
is really a suboptimal fit.

-- 
Cheers,

David / dhildenb