linux-kernel - Re: [RFC v2 0/2] add THP_HUGE_ZERO_PAGE

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Message-ID: <6894a8b1-a1a7-4a35-8193-68df3340f0ad@redhat.com>
Date: Thu, 22 May 2025 14:04:19 +0200
From: David Hildenbrand <david@...hat.com>
To: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>,
 Mike Rapoport <rppt@...nel.org>
Cc: Pankaj Raghav <p.raghav@...sung.com>,
 Suren Baghdasaryan <surenb@...gle.com>, Vlastimil Babka <vbabka@...e.cz>,
 Ryan Roberts <ryan.roberts@....com>, Michal Hocko <mhocko@...e.com>,
 Thomas Gleixner <tglx@...utronix.de>, Nico Pache <npache@...hat.com>,
 Dev Jain <dev.jain@....com>, 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>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>, Jens Axboe <axboe@...nel.dk>,
 linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 "Darrick J . Wong" <djwong@...nel.org>, gost.dev@...sung.com, hch@....de,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, willy@...radead.org,
 x86@...nel.org, mcgrof@...nel.org
Subject: Re: [RFC v2 0/2] add THP_HUGE_ZERO_PAGE_ALWAYS config option

On 22.05.25 14:00, Pankaj Raghav (Samsung) wrote:
> Hi Mike,
> 
>>> Add a config option THP_HUGE_ZERO_PAGE_ALWAYS that will always allocate
>>> the huge_zero_folio, and it will never be freed. This makes using the
>>> huge_zero_folio without having to pass any mm struct and a call to put_folio
>>> in the destructor.
>>
>> I don't think this config option should be tied to THP. It's perfectly
>> sensible to have a configuration with HUGETLB and without THP.
>>   
> 
> Hmm, that makes sense. You mean something like this (untested):
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 2e1527580746..d447a9b9eb7d 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -151,8 +151,8 @@ config X86
>          select ARCH_WANT_OPTIMIZE_DAX_VMEMMAP   if X86_64
>          select ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP       if X86_64
>          select ARCH_WANT_HUGETLB_VMEMMAP_PREINIT if X86_64
> +       select ARCH_WANTS_HUGE_ZERO_PAGE_ALWAYS if X86_64
>          select ARCH_WANTS_THP_SWAP              if X86_64
> -       select ARCH_WANTS_THP_ZERO_PAGE_ALWAYS  if X86_64
>          select ARCH_HAS_PARANOID_L1D_FLUSH
>          select BUILDTIME_TABLE_SORT
>          select CLKEVT_I8253
> diff --git a/mm/Kconfig b/mm/Kconfig
> index a2994e7d55ba..83a5b95a2286 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -823,9 +823,19 @@ config ARCH_WANT_GENERAL_HUGETLB
>   config ARCH_WANTS_THP_SWAP
>          def_bool n
>   
> -config ARCH_WANTS_THP_ZERO_PAGE_ALWAYS
> +config ARCH_WANTS_HUGE_ZERO_PAGE_ALWAYS
>          def_bool n
>   
> +config HUGE_ZERO_PAGE_ALWAYS

Likely something like

PMD_ZERO_PAGE

Will be a lot clearer.

 > +       def_bool y> +       depends on HUGETLB_PAGE && 
ARCH_WANTS_HUGE_ZERO_PAGE_ALWAYS

I suspect it should then also be independent of HUGETLB_PAGE?

> +       help
> +         Typically huge_zero_folio, which is a huge 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.

I assume that code then has to live in mm/memory.c ?

-- 
Cheers,

David / dhildenb