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. Add a config option STATIC_PMD_ZERO_PAGE that will always allocate the huge_zero_folio in .bss, and it will never be freed. This makes using the huge_zero_folio without having to pass any mm struct and call put_folio in the destructor. As STATIC_PMD_ZERO_PAGE does not depend on THP, declare huge_zero_folio and huge_zero_pfn outside of the THP ifdef. It can only be enabled from x86_64, but it is an optional config. We could expand it more architectures in the future. [1] https://lore.kernel.org/linux-xfs/20231027051847.GA7885@xxxxxx/ [2] https://lore.kernel.org/linux-xfs/ZitIK5OnR7ZNY0IG@xxxxxxxxxxxxx/ Suggested-by: David Hildenbrand <david@xxxxxxxxxx> Signed-off-by: Pankaj Raghav <p.raghav@xxxxxxxxxxx> --- Questions: - Can we call __split_huge_zero_page_pmd() on static PMD page? arch/x86/Kconfig | 1 + arch/x86/include/asm/pgtable.h | 8 ++++++++ arch/x86/kernel/head_64.S | 8 ++++++++ include/linux/mm.h | 16 +++++++++++++++- mm/Kconfig | 13 +++++++++++++ mm/huge_memory.c | 24 ++++++++++++++++++++---- mm/memory.c | 19 +++++++++++++++++++ 7 files changed, 84 insertions(+), 5 deletions(-) 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/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h index 774430c3abff..7013a7d26da5 100644 --- a/arch/x86/include/asm/pgtable.h +++ b/arch/x86/include/asm/pgtable.h @@ -47,6 +47,14 @@ void ptdump_walk_user_pgd_level_checkwx(void); #define debug_checkwx_user() do { } while (0) #endif +#ifdef CONFIG_STATIC_PMD_ZERO_PAGE +/* + * PMD_ZERO_PAGE is a global shared PMD page that is always zero. + */ +extern unsigned long empty_pmd_zero_page[(PMD_SIZE) / sizeof(unsigned long)] + __visible; +#endif + /* * ZERO_PAGE is a global shared page that is always zero: used * for zero-mapped memory areas etc.. diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S index 3e9b3a3bd039..86aaa53fd619 100644 --- a/arch/x86/kernel/head_64.S +++ b/arch/x86/kernel/head_64.S @@ -714,6 +714,14 @@ EXPORT_SYMBOL(phys_base) #include "../xen/xen-head.S" __PAGE_ALIGNED_BSS + +#ifdef CONFIG_STATIC_PMD_ZERO_PAGE +SYM_DATA_START_PAGE_ALIGNED(empty_pmd_zero_page) + .skip PMD_SIZE +SYM_DATA_END(empty_pmd_zero_page) +EXPORT_SYMBOL(empty_pmd_zero_page) +#endif + SYM_DATA_START_PAGE_ALIGNED(empty_zero_page) .skip PAGE_SIZE SYM_DATA_END(empty_zero_page) diff --git a/include/linux/mm.h b/include/linux/mm.h index c8fbeaacf896..b20d60d68b3c 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -4018,10 +4018,10 @@ static inline bool vma_is_special_huge(const struct vm_area_struct *vma) #endif /* CONFIG_TRANSPARENT_HUGEPAGE || CONFIG_HUGETLBFS */ -#ifdef CONFIG_TRANSPARENT_HUGEPAGE extern struct folio *huge_zero_folio; extern unsigned long huge_zero_pfn; +#ifdef CONFIG_TRANSPARENT_HUGEPAGE static inline bool is_huge_zero_folio(const struct folio *folio) { return READ_ONCE(huge_zero_folio) == folio; @@ -4032,9 +4032,23 @@ static inline bool is_huge_zero_pmd(pmd_t pmd) return pmd_present(pmd) && READ_ONCE(huge_zero_pfn) == pmd_pfn(pmd); } +#ifdef CONFIG_STATIC_PMD_ZERO_PAGE +static inline struct folio *mm_get_huge_zero_folio(struct mm_struct *mm) +{ + return READ_ONCE(huge_zero_folio); +} + +static inline void mm_put_huge_zero_folio(struct mm_struct *mm) +{ + return; +} + +#else struct folio *mm_get_huge_zero_folio(struct mm_struct *mm); void mm_put_huge_zero_folio(struct mm_struct *mm); +#endif /* CONFIG_STATIC_PMD_ZERO_PAGE */ + #else static inline bool is_huge_zero_folio(const struct folio *folio) { 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 + 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 a PMD sized zero page in .bss and huge_zero_folio will + use it instead allocating dynamically. + Not suitable for memory constrained systems. + menuconfig TRANSPARENT_HUGEPAGE bool "Transparent Hugepage Support" depends on HAVE_ARCH_TRANSPARENT_HUGEPAGE && !PREEMPT_RT diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 101b67ab2eb6..c12ca7134e88 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -75,9 +75,6 @@ static unsigned long deferred_split_scan(struct shrinker *shrink, struct shrink_control *sc); static bool split_underused_thp = true; -static atomic_t huge_zero_refcount; -struct folio *huge_zero_folio __read_mostly; -unsigned long huge_zero_pfn __read_mostly = ~0UL; unsigned long huge_anon_orders_always __read_mostly; unsigned long huge_anon_orders_madvise __read_mostly; unsigned long huge_anon_orders_inherit __read_mostly; @@ -208,6 +205,23 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma, return orders; } +#ifdef CONFIG_STATIC_PMD_ZERO_PAGE +static int huge_zero_page_shrinker_init(void) +{ + return 0; +} + +static void huge_zero_page_shrinker_exit(void) +{ + return; +} +#else + +static struct shrinker *huge_zero_page_shrinker; +static atomic_t huge_zero_refcount; +struct folio *huge_zero_folio __read_mostly; +unsigned long huge_zero_pfn __read_mostly = ~0UL; + static bool get_huge_zero_page(void) { struct folio *zero_folio; @@ -288,7 +302,6 @@ static unsigned long shrink_huge_zero_page_scan(struct shrinker *shrink, return 0; } -static struct shrinker *huge_zero_page_shrinker; static int huge_zero_page_shrinker_init(void) { huge_zero_page_shrinker = shrinker_alloc(0, "thp-zero"); @@ -307,6 +320,7 @@ static void huge_zero_page_shrinker_exit(void) return; } +#endif #ifdef CONFIG_SYSFS static ssize_t enabled_show(struct kobject *kobj, @@ -2843,6 +2857,8 @@ static void __split_huge_zero_page_pmd(struct vm_area_struct *vma, pte_t *pte; int i; + // FIXME: can this be called with static zero page? + VM_BUG_ON(IS_ENABLED(CONFIG_STATIC_PMD_ZERO_PAGE)); /* * Leave pmd empty until pte is filled note that it is fine to delay * notification until mmu_notifier_invalidate_range_end() as we are diff --git a/mm/memory.c b/mm/memory.c index 8eba595056fe..77721f5ae043 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -159,6 +159,25 @@ static int __init init_zero_pfn(void) } early_initcall(init_zero_pfn); +#ifdef CONFIG_STATIC_PMD_ZERO_PAGE +struct folio *huge_zero_folio __read_mostly; +unsigned long huge_zero_pfn __read_mostly = ~0UL; + +static int __init init_pmd_zero_pfn(void) +{ + huge_zero_folio = virt_to_folio(empty_pmd_zero_page); + huge_zero_pfn = page_to_pfn(virt_to_page(empty_pmd_zero_page)); + + __folio_set_head(huge_zero_folio); + prep_compound_head((struct page *)huge_zero_folio, PMD_ORDER); + /* Ensure zero folio won't have large_rmappable flag set. */ + folio_clear_large_rmappable(huge_zero_folio); + + return 0; +} +early_initcall(init_pmd_zero_pfn); +#endif + void mm_trace_rss_stat(struct mm_struct *mm, int member) { trace_rss_stat(mm, member); -- 2.49.0
