The patch titled
Subject: mm/mprotect: do not flush when not required architecturally
has been added to the -mm tree. Its filename is
mm-mprotect-do-not-flush-when-not-required-architecturally.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/mm-mprotect-do-not-flush-when-not-required-architecturally.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/mm-mprotect-do-not-flush-when-not-required-architecturally.patch
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------------------------------------------------------
From: Nadav Amit <namit@xxxxxxxxxx>
Subject: mm/mprotect: do not flush when not required architecturally
Currently, using mprotect() to unprotect a memory region or uffd to
unprotect a memory region causes a TLB flush. However, in such cases the
PTE is often not modified (i.e., remain RO) and therefore not TLB flush is
needed.
Add an arch-specific pte_needs_flush() which tells whether a TLB flush is
needed based on the old PTE and the new one. Implement an x86
pte_needs_flush().
Always flush the TLB when it is architecturally needed even when skipping
a TLB flush might only result in a spurious page-faults by skipping the
flush.
Even with such conservative manner, we can in the future further refine
the checks to test whether a PTE is present by only considering the
architectural _PAGE_PRESENT flag instead of {pte|pmd}_preesnt(). For not
be careful and use the latter.
Link: https://lkml.kernel.org/r/20220401180821.1986781-3-namit@xxxxxxxxxx
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
Cc: Nick Piggin <npiggin@xxxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
arch/x86/include/asm/pgtable_types.h | 2
arch/x86/include/asm/tlbflush.h | 97 +++++++++++++++++++++++++
include/asm-generic/tlb.h | 14 +++
mm/huge_memory.c | 9 +-
mm/mprotect.c | 3
5 files changed, 120 insertions(+), 5 deletions(-)
--- a/arch/x86/include/asm/pgtable_types.h~mm-mprotect-do-not-flush-when-not-required-architecturally
+++ a/arch/x86/include/asm/pgtable_types.h
@@ -115,9 +115,11 @@
#if defined(CONFIG_X86_64) || defined(CONFIG_X86_PAE)
#define _PAGE_NX (_AT(pteval_t, 1) << _PAGE_BIT_NX)
#define _PAGE_DEVMAP (_AT(u64, 1) << _PAGE_BIT_DEVMAP)
+#define _PAGE_SOFTW4 (_AT(pteval_t, 1) << _PAGE_BIT_SOFTW4)
#else
#define _PAGE_NX (_AT(pteval_t, 0))
#define _PAGE_DEVMAP (_AT(pteval_t, 0))
+#define _PAGE_SOFTW4 (_AT(pteval_t, 0))
#endif
#define _PAGE_PROTNONE (_AT(pteval_t, 1) << _PAGE_BIT_PROTNONE)
--- a/arch/x86/include/asm/tlbflush.h~mm-mprotect-do-not-flush-when-not-required-architecturally
+++ a/arch/x86/include/asm/tlbflush.h
@@ -259,6 +259,103 @@ static inline void arch_tlbbatch_add_mm(
extern void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch);
+static inline bool pte_flags_need_flush(unsigned long oldflags,
+ unsigned long newflags,
+ bool ignore_access)
+{
+ /*
+ * Flags that require a flush when cleared but not when they are set.
+ * Only include flags that would not trigger spurious page-faults.
+ * Non-present entries are not cached. Hardware would set the
+ * dirty/access bit if needed without a fault.
+ */
+ const pteval_t flush_on_clear = _PAGE_DIRTY | _PAGE_PRESENT |
+ _PAGE_ACCESSED;
+ const pteval_t software_flags = _PAGE_SOFTW1 | _PAGE_SOFTW2 |
+ _PAGE_SOFTW3 | _PAGE_SOFTW4;
+ const pteval_t flush_on_change = _PAGE_RW | _PAGE_USER | _PAGE_PWT |
+ _PAGE_PCD | _PAGE_PSE | _PAGE_GLOBAL | _PAGE_PAT |
+ _PAGE_PAT_LARGE | _PAGE_PKEY_BIT0 | _PAGE_PKEY_BIT1 |
+ _PAGE_PKEY_BIT2 | _PAGE_PKEY_BIT3 | _PAGE_NX;
+ unsigned long diff = oldflags ^ newflags;
+
+ BUILD_BUG_ON(flush_on_clear & software_flags);
+ BUILD_BUG_ON(flush_on_clear & flush_on_change);
+ BUILD_BUG_ON(flush_on_change & software_flags);
+
+ /* Ignore software flags */
+ diff &= ~software_flags;
+
+ if (ignore_access)
+ diff &= ~_PAGE_ACCESSED;
+
+ /*
+ * Did any of the 'flush_on_clear' flags was clleared set from between
+ * 'oldflags' and 'newflags'?
+ */
+ if (diff & oldflags & flush_on_clear)
+ return true;
+
+ /* Flush on modified flags. */
+ if (diff & flush_on_change)
+ return true;
+
+ /* Ensure there are no flags that were left behind */
+ if (IS_ENABLED(CONFIG_DEBUG_VM) &&
+ (diff & ~(flush_on_clear | software_flags | flush_on_change))) {
+ VM_WARN_ON_ONCE(1);
+ return true;
+ }
+
+ return false;
+}
+
+/*
+ * pte_needs_flush() checks whether permissions were demoted and require a
+ * flush. It should only be used for userspace PTEs.
+ */
+static inline bool pte_needs_flush(pte_t oldpte, pte_t newpte)
+{
+ /* !PRESENT -> * ; no need for flush */
+ if (!(pte_flags(oldpte) & _PAGE_PRESENT))
+ return false;
+
+ /* PFN changed ; needs flush */
+ if (pte_pfn(oldpte) != pte_pfn(newpte))
+ return true;
+
+ /*
+ * check PTE flags; ignore access-bit; see comment in
+ * ptep_clear_flush_young().
+ */
+ return pte_flags_need_flush(pte_flags(oldpte), pte_flags(newpte),
+ true);
+}
+#define pte_needs_flush pte_needs_flush
+
+/*
+ * huge_pmd_needs_flush() checks whether permissions were demoted and require a
+ * flush. It should only be used for userspace huge PMDs.
+ */
+static inline bool huge_pmd_needs_flush(pmd_t oldpmd, pmd_t newpmd)
+{
+ /* !PRESENT -> * ; no need for flush */
+ if (!(pmd_flags(oldpmd) & _PAGE_PRESENT))
+ return false;
+
+ /* PFN changed ; needs flush */
+ if (pmd_pfn(oldpmd) != pmd_pfn(newpmd))
+ return true;
+
+ /*
+ * check PMD flags; do not ignore access-bit; see
+ * pmdp_clear_flush_young().
+ */
+ return pte_flags_need_flush(pmd_flags(oldpmd), pmd_flags(newpmd),
+ false);
+}
+#define huge_pmd_needs_flush huge_pmd_needs_flush
+
#endif /* !MODULE */
static inline void __native_tlb_flush_global(unsigned long cr4)
--- a/include/asm-generic/tlb.h~mm-mprotect-do-not-flush-when-not-required-architecturally
+++ a/include/asm-generic/tlb.h
@@ -654,6 +654,20 @@ static inline void tlb_flush_p4d_range(s
} while (0)
#endif
+#ifndef pte_needs_flush
+static inline bool pte_needs_flush(pte_t oldpte, pte_t newpte)
+{
+ return true;
+}
+#endif
+
+#ifndef huge_pmd_needs_flush
+static inline bool huge_pmd_needs_flush(pmd_t oldpmd, pmd_t newpmd)
+{
+ return true;
+}
+#endif
+
#endif /* CONFIG_MMU */
#endif /* _ASM_GENERIC__TLB_H */
--- a/mm/huge_memory.c~mm-mprotect-do-not-flush-when-not-required-architecturally
+++ a/mm/huge_memory.c
@@ -1715,7 +1715,7 @@ int change_huge_pmd(struct mmu_gather *t
{
struct mm_struct *mm = vma->vm_mm;
spinlock_t *ptl;
- pmd_t entry;
+ pmd_t oldpmd, entry;
bool preserve_write;
int ret;
bool prot_numa = cp_flags & MM_CP_PROT_NUMA;
@@ -1804,9 +1804,9 @@ int change_huge_pmd(struct mmu_gather *t
* pmdp_invalidate() is required to make sure we don't miss
* dirty/young flags set by hardware.
*/
- entry = pmdp_invalidate(vma, addr, pmd);
+ oldpmd = pmdp_invalidate(vma, addr, pmd);
- entry = pmd_modify(entry, newprot);
+ entry = pmd_modify(oldpmd, newprot);
if (preserve_write)
entry = pmd_mk_savedwrite(entry);
if (uffd_wp) {
@@ -1823,7 +1823,8 @@ int change_huge_pmd(struct mmu_gather *t
ret = HPAGE_PMD_NR;
set_pmd_at(mm, addr, pmd, entry);
- tlb_flush_pmd_range(tlb, addr, HPAGE_PMD_SIZE);
+ if (huge_pmd_needs_flush(oldpmd, entry))
+ tlb_flush_pmd_range(tlb, addr, HPAGE_PMD_SIZE);
BUG_ON(vma_is_anonymous(vma) && !preserve_write && pmd_write(entry));
unlock:
--- a/mm/mprotect.c~mm-mprotect-do-not-flush-when-not-required-architecturally
+++ a/mm/mprotect.c
@@ -152,7 +152,8 @@ static unsigned long change_pte_range(st
ptent = pte_mkwrite(ptent);
}
ptep_modify_prot_commit(vma, addr, pte, oldpte, ptent);
- tlb_flush_pte_range(tlb, addr, PAGE_SIZE);
+ if (pte_needs_flush(oldpte, ptent))
+ tlb_flush_pte_range(tlb, addr, PAGE_SIZE);
pages++;
} else if (is_swap_pte(oldpte)) {
swp_entry_t entry = pte_to_swp_entry(oldpte);
_
Patches currently in -mm which might be from namit@xxxxxxxxxx are
userfaultfd-mark-uffd_wp-regardless-of-vm_write-flag.patch
mm-mprotect-use-mmu_gather.patch
mm-mprotect-do-not-flush-when-not-required-architecturally.patch
mm-avoid-unnecessary-flush-on-change_huge_pmd.patch
