linux-kernel - Re: [RFC 2/3] mm: add STATIC_PMD_ZERO

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Message-ID: <31dd1efc-ccf3-4908-a06f-20dabac86ce1@redhat.com>
Date: Wed, 28 May 2025 22:36:33 +0200
From: David Hildenbrand <david@...hat.com>
To: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>,
 Dave Hansen <dave.hansen@...el.com>
Cc: Pankaj Raghav <p.raghav@...sung.com>,
 Suren Baghdasaryan <surenb@...gle.com>, Ryan Roberts <ryan.roberts@....com>,
 Vlastimil Babka <vbabka@...e.cz>, Baolin Wang
 <baolin.wang@...ux.alibaba.com>, Borislav Petkov <bp@...en8.de>,
 Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>,
 Zi Yan <ziy@...dia.com>, Mike Rapoport <rppt@...nel.org>,
 Dave Hansen <dave.hansen@...ux.intel.com>, Michal Hocko <mhocko@...e.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Thomas Gleixner <tglx@...utronix.de>, Nico Pache <npache@...hat.com>,
 Dev Jain <dev.jain@....com>, "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, linux-block@...r.kernel.org, willy@...radead.org,
 x86@...nel.org, linux-fsdevel@...r.kernel.org,
 "Darrick J . Wong" <djwong@...nel.org>, mcgrof@...nel.org,
 gost.dev@...sung.com, hch@....de
Subject: Re: [RFC 2/3] mm: add STATIC_PMD_ZERO_PAGE config option

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