From: Nadav Amit Subject: mm: avoid unnecessary flush on change_huge_pmd() Calls to change_protection_range() on THP can trigger, at least on x86, two TLB flushes for one page: one immediately, when pmdp_invalidate() is called by change_huge_pmd(), and then another one later (that can be batched) when change_protection_range() finishes. The first TLB flush is only necessary to prevent the dirty bit (and with a lesser importance the access bit) from changing while the PTE is modified. However, this is not necessary as the x86 CPUs set the dirty-bit atomically with an additional check that the PTE is (still) present. One caveat is Intel's Knights Landing that has a bug and does not do so. Leverage this behavior to eliminate the unnecessary TLB flush in change_huge_pmd(). Introduce a new arch specific pmdp_invalidate_ad() that only invalidates the access and dirty bit from further changes. Link: https://lkml.kernel.org/r/20220401180821.1986781-4-namit@vmware.com Signed-off-by: Nadav Amit Cc: Andrea Arcangeli Cc: Andrew Cooper Cc: Andy Lutomirski Cc: Dave Hansen Cc: Peter Xu Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: Will Deacon Cc: Yu Zhao Cc: Nick Piggin Signed-off-by: Andrew Morton --- arch/x86/include/asm/pgtable.h | 5 +++++ arch/x86/mm/pgtable.c | 10 ++++++++++ include/linux/pgtable.h | 20 ++++++++++++++++++++ mm/huge_memory.c | 4 ++-- mm/pgtable-generic.c | 8 ++++++++ 5 files changed, 45 insertions(+), 2 deletions(-) --- a/arch/x86/include/asm/pgtable.h~mm-avoid-unnecessary-flush-on-change_huge_pmd +++ a/arch/x86/include/asm/pgtable.h @@ -1168,6 +1168,11 @@ static inline pmd_t pmdp_establish(struc } } #endif + +#define __HAVE_ARCH_PMDP_INVALIDATE_AD +extern pmd_t pmdp_invalidate_ad(struct vm_area_struct *vma, + unsigned long address, pmd_t *pmdp); + /* * Page table pages are page-aligned. The lower half of the top * level is used for userspace and the top half for the kernel. --- a/arch/x86/mm/pgtable.c~mm-avoid-unnecessary-flush-on-change_huge_pmd +++ a/arch/x86/mm/pgtable.c @@ -608,6 +608,16 @@ int pmdp_clear_flush_young(struct vm_are return young; } + +pmd_t pmdp_invalidate_ad(struct vm_area_struct *vma, unsigned long address, + pmd_t *pmdp) +{ + /* + * No flush is necessary. Once an invalid PTE is established, the PTE's + * access and dirty bits cannot be updated. + */ + return pmdp_establish(vma, address, pmdp, pmd_mkinvalid(*pmdp)); +} #endif /** --- a/include/linux/pgtable.h~mm-avoid-unnecessary-flush-on-change_huge_pmd +++ a/include/linux/pgtable.h @@ -570,6 +570,26 @@ extern pmd_t pmdp_invalidate(struct vm_a pmd_t *pmdp); #endif +#ifndef __HAVE_ARCH_PMDP_INVALIDATE_AD + +/* + * pmdp_invalidate_ad() invalidates the PMD while changing a transparent + * hugepage mapping in the page tables. This function is similar to + * pmdp_invalidate(), but should only be used if the access and dirty bits would + * not be cleared by the software in the new PMD value. The function ensures + * that hardware changes of the access and dirty bits updates would not be lost. + * + * Doing so can allow in certain architectures to avoid a TLB flush in most + * cases. Yet, another TLB flush might be necessary later if the PMD update + * itself requires such flush (e.g., if protection was set to be stricter). Yet, + * even when a TLB flush is needed because of the update, the caller may be able + * to batch these TLB flushing operations, so fewer TLB flush operations are + * needed. + */ +extern pmd_t pmdp_invalidate_ad(struct vm_area_struct *vma, + unsigned long address, pmd_t *pmdp); +#endif + #ifndef __HAVE_ARCH_PTE_SAME static inline int pte_same(pte_t pte_a, pte_t pte_b) { --- a/mm/huge_memory.c~mm-avoid-unnecessary-flush-on-change_huge_pmd +++ a/mm/huge_memory.c @@ -1801,10 +1801,10 @@ int change_huge_pmd(struct mmu_gather *t * The race makes MADV_DONTNEED miss the huge pmd and don't clear it * which may break userspace. * - * pmdp_invalidate() is required to make sure we don't miss + * pmdp_invalidate_ad() is required to make sure we don't miss * dirty/young flags set by hardware. */ - oldpmd = pmdp_invalidate(vma, addr, pmd); + oldpmd = pmdp_invalidate_ad(vma, addr, pmd); entry = pmd_modify(oldpmd, newprot); if (preserve_write) --- a/mm/pgtable-generic.c~mm-avoid-unnecessary-flush-on-change_huge_pmd +++ a/mm/pgtable-generic.c @@ -201,6 +201,14 @@ pmd_t pmdp_invalidate(struct vm_area_str } #endif +#ifndef __HAVE_ARCH_PMDP_INVALIDATE_AD +pmd_t pmdp_invalidate_ad(struct vm_area_struct *vma, unsigned long address, + pmd_t *pmdp) +{ + return pmdp_invalidate(vma, address, pmdp); +} +#endif + #ifndef pmdp_collapse_flush pmd_t pmdp_collapse_flush(struct vm_area_struct *vma, unsigned long address, pmd_t *pmdp) _