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

["Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>]

On Fri, Aug 01, 2025 at 05:49:10PM +0200, David Hildenbrand wrote:
> 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."

Sounds good.
> 
> > 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.
> 
Yeah, that makes sense. Let's go with try it once for now.

> 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?
Yeah, I can fold this in the next version. I guess WARN_ON_ONCE already
adds an unlikely to the conditition which is appropriate.

> 
> 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,
> 
--
Pankaj