linux-kernel - [PATCH v2 33/35] arm64/mm: attempt speculative mm faults first

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Date:   Fri, 28 Jan 2022 05:10:04 -0800
From:   Michel Lespinasse <michel@...pinasse.org>
To:     Linux-MM <linux-mm@...ck.org>, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     kernel-team@...com, Laurent Dufour <ldufour@...ux.ibm.com>,
        Jerome Glisse <jglisse@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Davidlohr Bueso <dave@...olabs.net>,
        Matthew Wilcox <willy@...radead.org>,
        Liam Howlett <liam.howlett@...cle.com>,
        Rik van Riel <riel@...riel.com>,
        Paul McKenney <paulmck@...nel.org>,
        Song Liu <songliubraving@...com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Minchan Kim <minchan@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Michel Lespinasse <michel@...pinasse.org>
Subject: [PATCH v2 33/35] arm64/mm: attempt speculative mm faults first

Attempt speculative mm fault handling first, and fall back to the
existing (non-speculative) code if that fails.

This follows the lines of the x86 speculative fault handling code,
but with some minor arch differences such as the way that the
VM_FAULT_BADACCESS case is handled.

Signed-off-by: Michel Lespinasse <michel@...pinasse.org>
---
 arch/arm64/mm/fault.c | 62 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 62 insertions(+)

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 77341b160aca..2598795f4e70 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -25,6 +25,7 @@
 #include <linux/perf_event.h>
 #include <linux/preempt.h>
 #include <linux/hugetlb.h>
+#include <linux/vm_event_item.h>
 
 #include <asm/acpi.h>
 #include <asm/bug.h>
@@ -524,6 +525,11 @@ static int __kprobes do_page_fault(unsigned long far, unsigned int esr,
 	unsigned long vm_flags;
 	unsigned int mm_flags = FAULT_FLAG_DEFAULT;
 	unsigned long addr = untagged_addr(far);
+#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+	struct vm_area_struct *vma;
+	struct vm_area_struct pvma;
+	unsigned long seq;
+#endif
 
 	if (kprobe_page_fault(regs, esr))
 		return 0;
@@ -574,6 +580,59 @@ static int __kprobes do_page_fault(unsigned long far, unsigned int esr,
 
 	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, addr);
 
+#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+	/*
+	 * No need to try speculative faults for kernel or
+	 * single threaded user space.
+	 */
+	if (!(mm_flags & FAULT_FLAG_USER) || atomic_read(&mm->mm_users) == 1)
+		goto no_spf;
+
+	count_vm_event(SPF_ATTEMPT);
+	seq = mmap_seq_read_start(mm);
+	if (seq & 1) {
+		count_vm_spf_event(SPF_ABORT_ODD);
+		goto spf_abort;
+	}
+	rcu_read_lock();
+	vma = __find_vma(mm, addr);
+	if (!vma || vma->vm_start > addr) {
+		rcu_read_unlock();
+		count_vm_spf_event(SPF_ABORT_UNMAPPED);
+		goto spf_abort;
+	}
+	if (!vma_is_anonymous(vma)) {
+		rcu_read_unlock();
+		count_vm_spf_event(SPF_ABORT_NO_SPECULATE);
+		goto spf_abort;
+	}
+	pvma = *vma;
+	rcu_read_unlock();
+	if (!mmap_seq_read_check(mm, seq, SPF_ABORT_VMA_COPY))
+		goto spf_abort;
+	vma = &pvma;
+	if (!(vma->vm_flags & vm_flags)) {
+		count_vm_spf_event(SPF_ABORT_ACCESS_ERROR);
+		goto spf_abort;
+	}
+	fault = do_handle_mm_fault(vma, addr & PAGE_MASK,
+			mm_flags | FAULT_FLAG_SPECULATIVE, seq, regs);
+
+	/* Quick path to respond to signals */
+	if (fault_signal_pending(fault, regs)) {
+		if (!user_mode(regs))
+			goto no_context;
+		return 0;
+	}
+	if (!(fault & VM_FAULT_RETRY))
+		goto done;
+
+spf_abort:
+	count_vm_event(SPF_ABORT);
+no_spf:
+
+#endif	/* CONFIG_SPECULATIVE_PAGE_FAULT */
+
 	/*
 	 * As per x86, we may deadlock here. However, since the kernel only
 	 * validly references user space from well defined areas of the code,
@@ -612,6 +671,9 @@ static int __kprobes do_page_fault(unsigned long far, unsigned int esr,
 		goto retry;
 	}
 	mmap_read_unlock(mm);
+#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+done:
+#endif
 
 	/*
 	 * Handle the "normal" (no error) case first.
-- 
2.20.1