[RFC 03/20] mm/mprotect: do not flush on permission promotion

Nadav Amit
2021-01-31T00:17:09Z

From: Nadav Amit <namit@vmware.com>

Currently, using mprotect() to unprotect a memory region or uffd to
unprotect a memory region causes a TLB flush. At least on x86, as
protection is promoted, no TLB flush is needed.

Add an arch-specific pte_may_need_flush() which tells whether a TLB
flush is needed based on the old PTE and the new one. Implement an x86
pte_may_need_flush().

For x86, besides the simple logic that PTE protection promotion or
changes of software bits does require a flush, also add logic that
considers the dirty-bit. If the dirty-bit is clear and write-protect is
set, no TLB flush is needed, as x86 updates the dirty-bit atomically
on write, and if the bit is clear, the PTE is reread.

Signed-off-by: Nadav Amit <namit@vmware.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will@kernel.org>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Nick Piggin <npiggin@gmail.com>
Cc: x86@kernel.org
---
arch/x86/include/asm/tlbflush.h | 44 +++++++++++++++++++++++++++++++++
include/asm-generic/tlb.h | 4 +++
mm/mprotect.c | 3 ++-
3 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
index 8c87a2e0b660..a617dc0a9b06 100644
--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -255,6 +255,50 @@ 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_may_need_flush(pte_t oldpte, pte_t newpte)
+{
+ const pteval_t ignore_mask = _PAGE_SOFTW1 | _PAGE_SOFTW2 |
+ _PAGE_SOFTW3 | _PAGE_ACCESSED;
+ const pteval_t enable_mask = _PAGE_RW | _PAGE_DIRTY | _PAGE_GLOBAL;
+ pteval_t oldval = pte_val(oldpte);
+ pteval_t newval = pte_val(newpte);
+ pteval_t diff = oldval ^ newval;
+ pteval_t disable_mask = 0;
+
+ if (IS_ENABLED(CONFIG_X86_64) || IS_ENABLED(CONFIG_X86_PAE))
+ disable_mask = _PAGE_NX;
+
+ /* new is non-present: need only if old is present */
+ if (pte_none(newpte))
+ return !pte_none(oldpte);
+
+ /*
+ * If, excluding the ignored bits, only RW and dirty are cleared and the
+ * old PTE does not have the dirty-bit set, we can avoid a flush. This
+ * is possible since x86 architecture set the dirty bit atomically while
+ * it caches the PTE in the TLB.
+ *
+ * The condition considers any change to RW and dirty as not requiring
+ * flush if the old PTE is not dirty or not writable for simplification
+ * of the code and to consider (unlikely) cases of changing dirty-bit of
+ * write-protected PTE.
+ */
+ if (!(diff & ~(_PAGE_RW | _PAGE_DIRTY | ignore_mask)) &&
+ (!(pte_dirty(oldpte) || !pte_write(oldpte))))
+ return false;
+
+ /*
+ * Any change of PFN and any flag other than those that we consider
+ * requires a flush (e.g., PAT, protection keys). To save flushes we do
+ * not consider the access bit as it is considered by the kernel as
+ * best-effort.
+ */
+ return diff & ((oldval & enable_mask) |
+ (newval & disable_mask) |
+ ~(enable_mask | disable_mask | ignore_mask));
+}
+#define pte_may_need_flush pte_may_need_flush
+
#endif /* !MODULE */

#endif /* _ASM_X86_TLBFLUSH_H */
diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
index eea113323468..c2deec0b6919 100644
--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -654,6 +654,10 @@ static inline void tlb_flush_p4d_range(struct mmu_gather *tlb,
} while (0)
#endif

+#ifndef pte_may_need_flush
+static inline bool pte_may_need_flush(pte_t oldpte, pte_t newpte) { return true; }
+#endif
+
#endif /* CONFIG_MMU */

#endif /* _ASM_GENERIC__TLB_H */
diff --git a/mm/mprotect.c b/mm/mprotect.c
index 632d5a677d3f..b7473d2c9a1f 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -139,7 +139,8 @@ 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_may_need_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