[PATCH] arm64: mm: Make flush_tlb_fix_spurious_fault() a no-op

[PATCH] arm64: mm: Make flush_tlb_fix_spurious_fault() a no-op

[Will Deacon]

Our use of broadcast TLB maintenance means that spurious page-faults
that have been handled already by another CPU do not require additional
TLB maintenance.

Make flush_tlb_fix_spurious_fault() a no-op and rely on the existing TLB
invalidation instead. Add an explicit flush_tlb_page() when making a page
dirty, as the TLB is permitted to cache the old read-only entry.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20200728092220.GA21800@willie-the-truck
Signed-off-by: Will Deacon <will@kernel.org>
---
 arch/arm64/include/asm/pgtable.h | 8 ++++++++
 arch/arm64/mm/fault.c            | 4 +++-
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index bc68da9f5706..02ad3105c14c 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -50,6 +50,14 @@ extern void __pgd_error(const char *file, int line, unsigned long val);
 	__flush_tlb_range(vma, addr, end, PUD_SIZE, false, 1)
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 
+/*
+ * Outside of a few very special situations (e.g. hibernation), we always
+ * use broadcast TLB invalidation instructions, therefore a spurious page
+ * fault on one CPU which has been handled concurrently by another CPU
+ * does not need to perform additional invalidation.
+ */
+#define flush_tlb_fix_spurious_fault(vma, address) do { } while (0)
+
 /*
  * ZERO_PAGE is a global shared page that is always zero: used
  * for zero-mapped memory areas etc..
diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index f07333e86c2f..a696a7921da4 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -218,7 +218,9 @@ int ptep_set_access_flags(struct vm_area_struct *vma,
 		pteval = cmpxchg_relaxed(&pte_val(*ptep), old_pteval, pteval);
 	} while (pteval != old_pteval);
 
-	flush_tlb_fix_spurious_fault(vma, address);
+	/* Invalidate a stale read-only entry */
+	if (dirty)
+		flush_tlb_page(vma, address);
 	return 1;
 }
 
-- 
2.28.0.709.gb0816b6eb0-goog

Re: [PATCH] arm64: mm: Make flush_tlb_fix_spurious_fault() a no-op

[Catalin Marinas]

On Wed, Sep 30, 2020 at 02:18:59PM +0100, Will Deacon wrote:
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index f07333e86c2f..a696a7921da4 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -218,7 +218,9 @@ int ptep_set_access_flags(struct vm_area_struct *vma,
>  		pteval = cmpxchg_relaxed(&pte_val(*ptep), old_pteval, pteval);
>  	} while (pteval != old_pteval);
>  
> -	flush_tlb_fix_spurious_fault(vma, address);
> +	/* Invalidate a stale read-only entry */
> +	if (dirty)
> +		flush_tlb_page(vma, address);
>  	return 1;

In my proposal I had a pte_accessible(pte) check instead of dirty here
since we may go for an old pte directly to a writable one and a TLBI
wouldn't be needed. Not that it matters from a performance perspective.

Either way,

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>