From: Nadav Amit <namit@xxxxxxxxxx>
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.
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
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: x86@xxxxxxxxxx
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
---
arch/x86/include/asm/pgtable_types.h | 2 +
arch/x86/include/asm/tlbflush.h | 121 +++++++++++++++++++++++++++
include/asm-generic/tlb.h | 14 ++++
mm/huge_memory.c | 9 +-
mm/mprotect.c | 3 +-
5 files changed, 144 insertions(+), 5 deletions(-)
diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
index 40497a9020c6..8668bc661026 100644
--- a/arch/x86/include/asm/pgtable_types.h
+++ b/arch/x86/include/asm/pgtable_types.h
@@ -110,9 +110,11 @@
#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)
diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
index 98fa0a114074..923d34a7214d 100644
--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -259,6 +259,127 @@ static inline void arch_tlbbatch_add_mm(struct arch_tlbflush_unmap_batch *batch,
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)
+{
+ unsigned long diff = oldflags ^ newflags;
+ pteval_t enable_mask, ignore_mask;
+
+ /* Ignore software bits */
+ ignore_mask = _PAGE_SOFTW1 | _PAGE_SOFTW2 | _PAGE_SOFTW3 | _PAGE_SOFTW4;
+
+ /*
+ * Flags that require a flush when cleared but not when they are set.
+ * We 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.
+ */
+ enable_mask = _PAGE_DIRTY | _PAGE_PRESENT;
+
+ if (ignore_access)
+ ignore_mask |= _PAGE_ACCESSED;
+ else
+ enable_mask |= _PAGE_ACCESSED;
+
+ /*
+ * Flags that indicate feature enabling are set in the old flags but
+ * not in the new flags; feature disabled; need flush.
+ */
+ if (diff & oldflags & enable_mask)
+ return true;
+
+ /*
+ * For other non-software flags (e.g., protection-keys, cachability,
+ * page-size, NX, write), we require a flush. For some cases (e.g.,
+ * protection keys) it is architecturally required, in others (e.g., NX)
+ * is done to avoid spurious page-faults.
+ */
+ if (diff & ~(ignore_mask | enable_mask))
+ 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)
+{
+ /*
+ * We first check whether only the new or old PTE is present. These
+ * tests are a more conservative than checking _PAGE_PRESENT as done by
+ * pte_flags_need_flush() since pte_present() also checks
+ * _PAGE_PROTNONE. Apparently _PAGE_PROTNONE can be ignored, since
+ * changes to _PAGE_PROTNONE are already followed by a flush. But for
+ * now, be conservative.
+ *
+ * In the future the first pte_present(oldpte) test can just check the
+ * _PAGE_PRESENT flag and the second one dropped.
+ */
+
+ /* !PRESENT -> * ; no need for flush */
+ if (!pte_present(oldpte))
+ return false;
+
+ /* PRESENT -> !PRESENT ; needs flush */
+ if (!pte_present(newpte))
+ return true;
+
+ /* 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)
+{
+ /*
+ * We first check whether only the new or old PMD is present. These
+ * tests are a more conservative than checking _PAGE_PRESENT as later
+ * done by pte_flags_need_flush(), since pte_present() also checks
+ * _PAGE_PROTNONE and _PAGE_PSE. Apparently, both _PAGE_PROTNONE and
+ * _PAGE_PSE can be ignored: changes to _PAGE_PROTNONE are followed by
+ * flush and _PAGE_PSE might only be set (without _PAGE_PRESENT) while
+ * the PMD's page-table lock is taken. But be conservative for now.
+ *
+ * In the future the first pte_present(oldpmd) test can just check the
+ * _PAGE_PRESENT flag and the second one dropped.
+ */
+
+ /* !PRESENT -> * ; no need for flush */
+ if (!pmd_present(oldpmd))
+ return false;
+
+ /* PRESENT -> !PRESENT ; needs flush */
+ if (!pmd_present(newpmd))
+ return true;
+
+ /* 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)
diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
index fd7feb5c7894..3a30e23fa35d 100644
--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -654,6 +654,20 @@ static inline void tlb_flush_p4d_range(struct mmu_gather *tlb,
} 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 */
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index d58a5b498011..51b0f3cb1ba0 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1698,7 +1698,7 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
{
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;
@@ -1784,9 +1784,9 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
* 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) {
@@ -1803,7 +1803,8 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
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:
diff --git a/mm/mprotect.c b/mm/mprotect.c
index ba3fc6d5ed2a..dd963e5da118 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -152,7 +152,8 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
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);
--
2.25.1
