linux-kernel - Re: [PATCH v2 0/5] add static PMD zero page support

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Message-ID: <ad876991-5736-4d4c-9f19-6076832d0c69@pankajraghav.com>
Date: Wed, 9 Jul 2025 10:03:51 +0200
From: Pankaj Raghav <kernel@...kajraghav.com>
To: Zi Yan <ziy@...dia.com>
Cc: Suren Baghdasaryan <surenb@...gle.com>,
 Ryan Roberts <ryan.roberts@....com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, Borislav Petkov <bp@...en8.de>,
 Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>,
 Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
 Dave Hansen <dave.hansen@...ux.intel.com>, Michal Hocko <mhocko@...e.com>,
 David Hildenbrand <david@...hat.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,
 willy@...radead.org, linux-mm@...ck.org, x86@...nel.org,
 linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 "Darrick J . Wong" <djwong@...nel.org>, mcgrof@...nel.org,
 gost.dev@...sung.com, hch@....de, Pankaj Raghav <p.raghav@...sung.com>
Subject: Re: [PATCH v2 0/5] add static PMD zero page support

Hi Zi,

>> Add a config option STATIC_PMD_ZERO_PAGE that will always allocate the huge_zero_folio via 
>> memblock, and it will never be freed.
> 
> Do the above users want a PMD sized zero page or a 2MB zero page? Because on systems with non 
> 4KB base page size, e.g., ARM64 with 64KB base page, PMD size is different. ARM64 with 64KB base 
> page has 512MB PMD sized pages. Having STATIC_PMD_ZERO_PAGE means losing half GB memory. I am 
> not sure if it is acceptable.
> 

That is a good point. My intial RFC patches allocated 2M instead of a PMD sized
page.

But later David wanted to reuse the memory we allocate here with huge_zero_folio. So
if this config is enabled, we simply just use the same pointer for huge_zero_folio.

Since that happened, I decided to go with PMD sized page.

This config is still opt in and I would expect the users with 64k page size systems to not enable
this.

But to make sure we don't enable this for those architecture, I could do a per-arch opt in with
something like this[1] that I did in my previous patch:

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 340e5468980e..c3a9d136ec0a 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -153,6 +153,7 @@ 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_HAS_STATIC_PMD_ZERO_PAGE	if X86_64
 	select ARCH_HAS_PARANOID_L1D_FLUSH
 	select ARCH_WANT_IRQS_OFF_ACTIVATE_MM
 	select BUILDTIME_TABLE_SORT

diff --git a/mm/Kconfig b/mm/Kconfig
index 781be3240e21..fd1c51995029 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -826,6 +826,19 @@ config ARCH_WANTS_THP_SWAP
 config MM_ID
 	def_bool n

+config ARCH_HAS_STATIC_PMD_ZERO_PAGE
+	def_bool n
+
+config STATIC_PMD_ZERO_PAGE
+	bool "Allocate a PMD page for zeroing"
+	depends on ARCH_HAS_STATIC_PMD_ZERO_PAGE
<snip>

Let me know your thoughts.

[1] https://lore.kernel.org/linux-mm/20250612105100.59144-4-p.raghav@samsung.com/#Z31mm:Kconfig
--
Pankaj