Message-ID: <3d935889-fcda-4345-bd57-6c7a84458493@redhat.com>
Date: Wed, 23 Jul 2025 11:06:05 +0200
From: David Hildenbrand <david@...hat.com>
To: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>,
 Suren Baghdasaryan <surenb@...gle.com>, Ryan Roberts <ryan.roberts@....com>,
 Mike Rapoport <rppt@...nel.org>, 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>, Vlastimil Babka <vbabka@...e.cz>,
 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>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, willy@...radead.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: [RFC 2/4] mm: add static huge zero folio

On 22.07.25 11:42, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@...sung.com>
> 
> There are many places in the kernel where we need to zeroout larger
> chunks but the maximum segment we can zeroout at a time by ZERO_PAGE
> is limited by PAGE_SIZE.
> 
> This is especially annoying in block devices and filesystems where we
> attach multiple ZERO_PAGEs to the bio in different bvecs. With multipage
> bvec support in block layer, it is much more efficient to send out
> larger zero pages as a part of single bvec.
> 
> This concern was raised during the review of adding LBS support to
> XFS[1][2].
> 
> Usually huge_zero_folio is allocated on demand, and it will be
> deallocated by the shrinker if there are no users of it left. At moment,
> huge_zero_folio infrastructure refcount is tied to the process lifetime
> that created it. This might not work for bio layer as the completions
> can be async and the process that created the huge_zero_folio might no
> longer be alive. And, one of the main point that came during discussion
> is to have something bigger than zero page as a drop-in replacement.
> 
> Add a config option STATIC_HUGE_ZERO_FOLIO that will always allocate
> the huge_zero_folio, and it will never drop the reference. This makes
> using the huge_zero_folio without having to pass any mm struct and does
> not tie the lifetime of the zero folio to anything, making it a drop-in
> replacement for ZERO_PAGE.
> 
> If STATIC_PMD_ZERO_PAGE config option is enabled, then
> mm_get_huge_zero_folio() will simply return this page instead of
> dynamically allocating a new PMD page.
> 
> This option can waste memory in small systems or systems with 64k base
> page size. So make it an opt-in and also add an option from individual
> architecture so that we don't enable this feature for larger base page
> size systems.
> 
> [1] https://lore.kernel.org/linux-xfs/20231027051847.GA7885@lst.de/
> [2] https://lore.kernel.org/linux-xfs/ZitIK5OnR7ZNY0IG@infradead.org/
> 
> Co-Developed-by: David Hildenbrand <david@...hat.com>

"Co-developed-by:"

And must be followed by

Signed-of-by: David Hildenbrand <david@...hat.com>

As mentioned to the cover letter: spaces vs. tabs.

> Signed-off-by: Pankaj Raghav <p.raghav@...sung.com>
> ---
>   arch/x86/Kconfig        |  1 +
>   include/linux/huge_mm.h | 16 ++++++++++++++++
>   mm/Kconfig              | 12 ++++++++++++
>   mm/huge_memory.c        | 28 ++++++++++++++++++++++++++++
>   4 files changed, 57 insertions(+)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 0ce86e14ab5e..8e2aa1887309 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_WANTS_STATIC_HUGE_ZERO_FOLIO if X86_64
>   	select ARCH_HAS_PARANOID_L1D_FLUSH
>   	select ARCH_WANT_IRQS_OFF_ACTIVATE_MM
>   	select BUILDTIME_TABLE_SORT
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 7748489fde1b..0ddd9c78f9f4 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -476,6 +476,7 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf);
>   
>   extern struct folio *huge_zero_folio;
>   extern unsigned long huge_zero_pfn;
> +extern atomic_t huge_zero_folio_is_static;
>   
>   static inline bool is_huge_zero_folio(const struct folio *folio)
>   {
> @@ -494,6 +495,16 @@ static inline bool is_huge_zero_pmd(pmd_t pmd)
>   
>   struct folio *mm_get_huge_zero_folio(struct mm_struct *mm);
>   void mm_put_huge_zero_folio(struct mm_struct *mm);
> +struct folio *__get_static_huge_zero_folio(void);
> +
> +static inline struct folio *get_static_huge_zero_folio(void)
> +{
> +       if (!IS_ENABLED(CONFIG_STATIC_HUGE_ZERO_FOLIO))
> +               return NULL;
> +       if (likely(atomic_read(&huge_zero_folio_is_static)))
> +               return huge_zero_folio;
 > +       return __get_static_huge_zero_folio();> +}
>   
>   static inline bool thp_migration_supported(void)
>   {
> @@ -685,6 +696,11 @@ static inline int change_huge_pud(struct mmu_gather *tlb,
>   {
>   	return 0;
>   }
> +
> +static inline struct folio *get_static_huge_zero_folio(void)
> +{
> +       return NULL;
> +}
>   #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>   
>   static inline int split_folio_to_list_to_order(struct folio *folio,
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 0287e8d94aea..14721171846f 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -835,6 +835,18 @@ config ARCH_WANT_GENERAL_HUGETLB
>   config ARCH_WANTS_THP_SWAP
>   	def_bool n
>   
> +config ARCH_WANTS_STATIC_HUGE_ZERO_FOLIO
> +	def_bool n
> +
> +config STATIC_HUGE_ZERO_FOLIO
> +	bool "Allocate a PMD sized folio for zeroing"
> +	depends on ARCH_WANTS_STATIC_HUGE_ZERO_FOLIO
> +	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
> +	  allocate huge_zero_folio but it will never free it.
> +	  Not suitable for memory constrained systems.

Maybe something like

"
Without this config enabled, the huge zero folio is allocated on demand 
and freed under memory pressure once no longer in use. To detect 
remaining users reliably, references to the huge zero folio must be 
tracked precisely, so it is commonly only available for mapping it into 
user page tables.

With this config enabled, the huge zero folio can also be used for other 
purposes that do not implement precise reference counting: it is still 
allocated on demand, but never freed, allowing for more wide-spread use,
for example, when performing I/O similar to the traditional shared 
zeropage."

Not suitable for memory constrained systems.
"

Should we make it clear that this is currently limited to THP configs?

depends on TRANSPARENT_HUGEPAGE

> +
>   config MM_ID
>   	def_bool n
>   
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 5d8365d1d3e9..6c890a1482f3 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -75,6 +75,8 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>   static bool split_underused_thp = true;
>   
>   static atomic_t huge_zero_refcount;
> +static atomic_t huge_zero_static_fail_count __read_mostly;
> +atomic_t huge_zero_folio_is_static __read_mostly;
>   struct folio *huge_zero_folio __read_mostly;
>   unsigned long huge_zero_pfn __read_mostly = ~0UL;
>   unsigned long huge_anon_orders_always __read_mostly;
> @@ -266,6 +268,32 @@ void mm_put_huge_zero_folio(struct mm_struct *mm)
>   		put_huge_zero_page();
>   }
>   
> +#ifdef CONFIG_STATIC_HUGE_ZERO_FOLIO
> +struct folio *__get_static_huge_zero_folio(void)

Do we want to play safe and have a

if (unlikely(!slab_is_available()))
	return NULL;

> +{
> +       /*
> +        * If we failed to allocate a huge zero folio multiple times,
> +        * just refrain from trying.
> +        */

Hmmm, I wonder if we want to retry "some time later" again. Meaning, 
we'd base it on the jiffies, maybe?

See print_bad_pte() for an example.

> +       if (atomic_read(&huge_zero_static_fail_count) > 2)
> +               return NULL;
> +

We could make some smart decision regarding totalram_pages() and just 
disable it. A bit tricky, we can do that as a follow-up.

> +       /*
> +        * Our raised reference will prevent the shrinker from ever having
> +        * success.
> +        */
> +       if (!get_huge_zero_page()) {
> +              atomic_inc(&huge_zero_static_fail_count);
> +               return NULL;
> +       }
> +
> +       if (atomic_cmpxchg(&huge_zero_folio_is_static, 0, 1) != 0)
> +               put_huge_zero_page();
> +
> +       return huge_zero_folio;
> +}
> +#endif /* CONFIG_STATIC_HUGE_ZERO_FOLIO */
> +
>   static unsigned long shrink_huge_zero_folio_count(struct shrinker *shrink,
>   						  struct shrink_control *sc)
>   {


-- 
Cheers,

David / dhildenb

Powered by blists - more mailing lists