linux-kernel - [PATCH v10 2/3] arm64: mm: implement arch_faults_on_old

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Message-Id: <20190930015740.84362-3-justin.he@arm.com>
Date:   Mon, 30 Sep 2019 09:57:39 +0800
From:   Jia He <justin.he@....com>
To:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        Marc Zyngier <maz@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Cc:     Punit Agrawal <punitagrawal@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>, hejianet@...il.com,
        Kaly Xin <Kaly.Xin@....com>, Jia He <justin.he@....com>
Subject: [PATCH v10 2/3] arm64: mm: implement arch_faults_on_old_pte() on arm64

On arm64 without hardware Access Flag, copying fromuser will fail because
the pte is old and cannot be marked young. So we always end up with zeroed
page after fork() + CoW for pfn mappings. we don't always have a
hardware-managed access flag on arm64.

Hence implement arch_faults_on_old_pte on arm64 to indicate that it might
cause page fault when accessing old pte.

Signed-off-by: Jia He <justin.he@....com>
Reviewed-by: Catalin Marinas <catalin.marinas@....com>
---
 arch/arm64/include/asm/pgtable.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 7576df00eb50..e96fb82f62de 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -885,6 +885,20 @@ static inline void update_mmu_cache(struct vm_area_struct *vma,
 #define phys_to_ttbr(addr)	(addr)
 #endif
 
+/*
+ * On arm64 without hardware Access Flag, copying from user will fail because
+ * the pte is old and cannot be marked young. So we always end up with zeroed
+ * page after fork() + CoW for pfn mappings. We don't always have a
+ * hardware-managed access flag on arm64.
+ */
+static inline bool arch_faults_on_old_pte(void)
+{
+	WARN_ON(preemptible());
+
+	return !cpu_has_hw_af();
+}
+#define arch_faults_on_old_pte arch_faults_on_old_pte
+
 #endif /* !__ASSEMBLY__ */
 
 #endif /* __ASM_PGTABLE_H */
-- 
2.17.1