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 THP_HUGE_ZERO_PAGE_ALWAYS that will always allocate the huge_zero_folio, 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. We can enable it by default for x86_64 where the PMD size is 2M. It is good compromise between the memory and efficiency. As a THP zero page might be wasteful for architectures with bigger page sizes, let's not enable it for them. [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> --- arch/x86/Kconfig | 1 + mm/Kconfig | 12 +++++++++ mm/huge_memory.c | 63 ++++++++++++++++++++++++++++++++++++++---------- 3 files changed, 63 insertions(+), 13 deletions(-) diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index 055204dc211d..2e1527580746 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -152,6 +152,7 @@ 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_THP_ZERO_PAGE_ALWAYS if X86_64 select ARCH_HAS_PARANOID_L1D_FLUSH select BUILDTIME_TABLE_SORT select CLKEVT_I8253 diff --git a/mm/Kconfig b/mm/Kconfig index bd08e151fa1b..a2994e7d55ba 100644 --- a/mm/Kconfig +++ b/mm/Kconfig @@ -823,6 +823,9 @@ config ARCH_WANT_GENERAL_HUGETLB config ARCH_WANTS_THP_SWAP def_bool n +config ARCH_WANTS_THP_ZERO_PAGE_ALWAYS + def_bool n + config MM_ID def_bool n @@ -895,6 +898,15 @@ config READ_ONLY_THP_FOR_FS support of file THPs will be developed in the next few release cycles. +config THP_ZERO_PAGE_ALWAYS + def_bool y + depends on TRANSPARENT_HUGEPAGE && ARCH_WANTS_THP_ZERO_PAGE_ALWAYS + help + Typically huge_zero_folio, which is a THP of zeroes, is allocated + on demand and deallocated when not in use. This option will always + allocate huge_zero_folio for zeroing and it is never deallocated. + Not suitable for memory constrained systems. + config NO_PAGE_MAPCOUNT bool "No per-page mapcount (EXPERIMENTAL)" help diff --git a/mm/huge_memory.c b/mm/huge_memory.c index d3e66136e41a..1a0556ca3839 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -247,9 +247,16 @@ static void put_huge_zero_page(void) BUG_ON(atomic_dec_and_test(&huge_zero_refcount)); } +/* + * If THP_ZERO_PAGE_ALWAYS is enabled, @mm can be NULL, i.e, the huge_zero_folio + * is not associated with any mm_struct. + */ struct folio *mm_get_huge_zero_folio(struct mm_struct *mm) { - if (test_bit(MMF_HUGE_ZERO_PAGE, &mm->flags)) + if (!IS_ENABLED(CONFIG_THP_ZERO_PAGE_ALWAYS) && !mm) + return NULL; + + if (IS_ENABLED(CONFIG_THP_ZERO_PAGE_ALWAYS) || test_bit(MMF_HUGE_ZERO_PAGE, &mm->flags)) return READ_ONCE(huge_zero_folio); if (!get_huge_zero_page()) @@ -263,6 +270,9 @@ struct folio *mm_get_huge_zero_folio(struct mm_struct *mm) void mm_put_huge_zero_folio(struct mm_struct *mm) { + if (IS_ENABLED(CONFIG_THP_ZERO_PAGE_ALWAYS)) + return; + if (test_bit(MMF_HUGE_ZERO_PAGE, &mm->flags)) put_huge_zero_page(); } @@ -274,14 +284,21 @@ static unsigned long shrink_huge_zero_page_count(struct shrinker *shrink, return atomic_read(&huge_zero_refcount) == 1 ? HPAGE_PMD_NR : 0; } +static void _put_huge_zero_folio(void) +{ + struct folio *zero_folio; + + zero_folio = xchg(&huge_zero_folio, NULL); + BUG_ON(zero_folio == NULL); + WRITE_ONCE(huge_zero_pfn, ~0UL); + folio_put(zero_folio); +} + 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); - BUG_ON(zero_folio == NULL); - WRITE_ONCE(huge_zero_pfn, ~0UL); - folio_put(zero_folio); + _put_huge_zero_folio(); return HPAGE_PMD_NR; } @@ -850,10 +867,6 @@ static inline void hugepage_exit_sysfs(struct kobject *hugepage_kobj) static int __init thp_shrinker_init(void) { - huge_zero_page_shrinker = shrinker_alloc(0, "thp-zero"); - if (!huge_zero_page_shrinker) - return -ENOMEM; - deferred_split_shrinker = shrinker_alloc(SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE | SHRINKER_NONSLAB, @@ -863,14 +876,21 @@ static int __init thp_shrinker_init(void) return -ENOMEM; } - huge_zero_page_shrinker->count_objects = shrink_huge_zero_page_count; - huge_zero_page_shrinker->scan_objects = shrink_huge_zero_page_scan; - shrinker_register(huge_zero_page_shrinker); - deferred_split_shrinker->count_objects = deferred_split_count; deferred_split_shrinker->scan_objects = deferred_split_scan; shrinker_register(deferred_split_shrinker); + if (IS_ENABLED(CONFIG_THP_ZERO_PAGE_ALWAYS)) + return 0; + + huge_zero_page_shrinker = shrinker_alloc(0, "thp-zero"); + if (!huge_zero_page_shrinker) + return -ENOMEM; + + huge_zero_page_shrinker->count_objects = shrink_huge_zero_page_count; + huge_zero_page_shrinker->scan_objects = shrink_huge_zero_page_scan; + shrinker_register(huge_zero_page_shrinker); + return 0; } @@ -880,6 +900,17 @@ static void __init thp_shrinker_exit(void) shrinker_free(deferred_split_shrinker); } +static int __init huge_zero_page_init(void) { + + if (!IS_ENABLED(CONFIG_THP_ZERO_PAGE_ALWAYS)) + return 0; + + if (!get_huge_zero_page()) { + return -ENOMEM; + } + return 0; +} + static int __init hugepage_init(void) { int err; @@ -903,6 +934,10 @@ static int __init hugepage_init(void) if (err) goto err_slab; + err = huge_zero_page_init(); + if (err) + goto err_huge_zero_page; + err = thp_shrinker_init(); if (err) goto err_shrinker; @@ -925,6 +960,8 @@ static int __init hugepage_init(void) err_khugepaged: thp_shrinker_exit(); err_shrinker: + _put_huge_zero_folio(); +err_huge_zero_page: khugepaged_destroy(); err_slab: hugepage_exit_sysfs(hugepage_kobj); -- 2.47.2
