Re: [RFC v2 2/4] mm: add static huge zero folio

[David Hildenbrand <david@redhat.com>]

On 24.07.25 16:49, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@samsung.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

"... will result in allocating the huge zero folio on first request, if not already allocated, and turn it static such that it can never get freed."

> 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_HUGE_ZERO_FOLIO 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@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> ---
>   arch/x86/Kconfig        |  1 +
>   include/linux/huge_mm.h | 18 ++++++++++++++++++
>   mm/Kconfig              | 21 +++++++++++++++++++++
>   mm/huge_memory.c        | 42 +++++++++++++++++++++++++++++++++++++++++
>   4 files changed, 82 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..78ebceb61d0e 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 +495,18 @@ 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 +698,11 @@ 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..e2132fcf2ccb 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -835,6 +835,27 @@ 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 && TRANSPARENT_HUGEPAGE
> +	help
> +	  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.
> +
>   config MM_ID
>   	def_bool n
>   
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 5d8365d1d3e9..c160c37f4d31 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -75,6 +75,7 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>   static bool split_underused_thp = true;
>   
>   static atomic_t huge_zero_refcount;
> +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 +267,47 @@ void mm_put_huge_zero_folio(struct mm_struct *mm)
>   		put_huge_zero_page();
>   }
>   
> +#ifdef CONFIG_STATIC_HUGE_ZERO_FOLIO
> +#define FAIL_COUNT_LIMIT 2
> +
> +struct folio *__get_static_huge_zero_folio(void)
> +{
> +	static unsigned long fail_count_clear_timer;
> +	static atomic_t huge_zero_static_fail_count __read_mostly;
> +
> +	if (unlikely(!slab_is_available()))
> +		return NULL;
> +
> +	/*
> +	 * If we failed to allocate a huge zero folio multiple times,
> +	 * just refrain from trying for one minute before retrying to get
> +	 * a reference again.
> +	 */

Is this "try twice" really worth it? Just try once, and if it fails, try only again in the future.

I guess we'll learn how that will behave in practice, and how we'll have to fine-tune it :)


In shrink_huge_zero_page_scan(), should we probably warn if something buggy happens?

Something like

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 2b4ea5a2ce7d2..b1109f8699a24 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -277,7 +277,11 @@ static unsigned long shrink_huge_zero_page_scan(struct shrinker *shrink,
                                        struct shrink_control *sc)
  {
         if (atomic_cmpxchg(&huge_zero_refcount, 1, 0) == 1) {
-               struct folio *zero_folio = xchg(&huge_zero_folio, NULL);
+               struct folio *zero_folio;
+
+               if (WARN_ON_ONCE(atomic_read(&huge_zero_folio_is_static)))
+                       return 0;
+               zero_folio = xchg(&huge_zero_folio, NULL);
                 BUG_ON(zero_folio == NULL);
                 WRITE_ONCE(huge_zero_pfn, ~0UL);
                 folio_put(zero_folio);


-- 
Cheers,

David / dhildenb