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From d28921b5f797829e4e676f7968ae688ef96b7992 Mon Sep 17 00:00:00 2001
From: Sebastien Boeuf <sebastien.boeuf@intel.com>
Date: Mon, 23 Jan 2017 15:08:55 -0800
Subject: [PATCH 152/154] x86: kvm: Notify host to release pages
In context of hypervisors managing several virtual machines, we
want those virtual machines to give the memory they used back to
the host when they don't need it anymore.
This patch introduces a new hypercall KVM_HC_RETURN_MEM, allowing
the guest kernel to notify the host kernel when such event occurs.
And relying on do_madvise() function that we have previously exported,
it issues a call to this function when it receives the new hypercall.
Use of do_madvise() with MADV_DONTNEED flag will allow the guest to
ask for a new page without going through a new hypercall. Instead,
it will be able to start using that memory again as it will get
faulted back in as a fresh new page. That's why do_madvise() is more
efficient than doing vm_unmap() to return some memory to the host.
This patch introduces also a new sysctl kvm_madv_instant_free,
allowing user to set MADV_FREE advice instead of MADV_DONTNEED.
Indeed, MADV_FREE saves more performances than using MADV_DONTNEED
because it does not zero the pages in case the memory has not been
freed by the kernel. This can happen when there was no need for the
kernel to get this memory back, meaning it was keeping those pages
in the right state to be re-used by the same application.
MADV_FREE being a very recent advice introduced in kernel 4.5, we
only want to enable it through a sysctl in case the user want to
use it.
Suggested-by: Arjan van de Ven <arjan.van.de.ven@intel.com>
Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>
---
arch/x86/kvm/x86.c | 17 +++++++++++++++++
include/linux/mm.h | 5 +++++
include/uapi/linux/kvm_para.h | 3 +++
kernel/sysctl.c | 7 +++++++
mm/Makefile | 2 +-
mm/kvm.c | 25 +++++++++++++++++++++++++
6 files changed, 58 insertions(+), 1 deletion(-)
create mode 100644 mm/kvm.c
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 582c75311f95..683a94dd5f03 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -46,6 +46,7 @@
#include <linux/user-return-notifier.h>
#include <linux/srcu.h>
#include <linux/slab.h>
+#include <linux/mm.h>
#include <linux/perf_event.h>
#include <linux/uaccess.h>
#include <linux/hash.h>
@@ -6019,6 +6020,19 @@ static void kvm_pv_kick_cpu_op(struct kvm *kvm, unsigned long flags, int apicid)
kvm_irq_delivery_to_apic(kvm, NULL, &lapic_irq, NULL);
}
+static int kvm_pv_return_mem_op(struct kvm *kvm, gpa_t gpa, size_t len)
+{
+ unsigned long start = gfn_to_hva(kvm, gpa_to_gfn(gpa));
+
+ if (len > KVM_MAX_RET_MEM_SIZE)
+ return KVM_EPERM;
+
+ if (kvm_is_error_hva(start + len))
+ return KVM_EFAULT;
+
+ return do_madvise(start, len, kvm_ret_mem_advice);
+}
+
void kvm_vcpu_deactivate_apicv(struct kvm_vcpu *vcpu)
{
vcpu->arch.apicv_active = false;
@@ -6065,6 +6079,9 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
kvm_pv_kick_cpu_op(vcpu->kvm, a0, a1);
ret = 0;
break;
+ case KVM_HC_RETURN_MEM:
+ ret = kvm_pv_return_mem_op(vcpu->kvm, a0, a1);
+ break;
default:
ret = -KVM_ENOSYS;
break;
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 925ec25f99a8..833f23d98baa 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2303,6 +2303,11 @@ extern bool process_shares_mm(struct task_struct *p, struct mm_struct *mm);
extern int sysctl_drop_caches;
int drop_caches_sysctl_handler(struct ctl_table *, int,
void __user *, size_t *, loff_t *);
+extern int sysctl_kvm_madv_instant_free;
+extern int kvm_ret_mem_advice;
+int kvm_madv_instant_free_sysctl_handler(struct ctl_table *table, int write,
+ void __user *buffer, size_t *length,
+ loff_t *ppos);
#endif
void drop_slab(void);
diff --git a/include/uapi/linux/kvm_para.h b/include/uapi/linux/kvm_para.h
index bf6cd7d5cac2..7d90f77d87d0 100644
--- a/include/uapi/linux/kvm_para.h
+++ b/include/uapi/linux/kvm_para.h
@@ -23,6 +23,9 @@
#define KVM_HC_MIPS_GET_CLOCK_FREQ 6
#define KVM_HC_MIPS_EXIT_VM 7
#define KVM_HC_MIPS_CONSOLE_OUTPUT 8
+#define KVM_HC_RETURN_MEM 10
+
+#define KVM_MAX_RET_MEM_SIZE (1 << 22) // 4MiB
/*
* hypercalls use architecture specific
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index c1095cdc0fe2..d8ae774fa042 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1398,6 +1398,13 @@ static struct ctl_table vm_table[] = {
.extra1 = &one,
.extra2 = &four,
},
+ {
+ .procname = "kvm_madv_instant_free",
+ .data = &sysctl_kvm_madv_instant_free,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = kvm_madv_instant_free_sysctl_handler,
+ },
#ifdef CONFIG_COMPACTION
{
.procname = "compact_memory",
diff --git a/mm/Makefile b/mm/Makefile
index 295bd7a..6455723 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -47,6 +47,8 @@ else
obj-y += bootmem.o
endif
+obj-y += kvm.o
+
obj-$(CONFIG_ADVISE_SYSCALLS) += fadvise.o
ifdef CONFIG_MMU
obj-$(CONFIG_ADVISE_SYSCALLS) += madvise.o
diff --git a/mm/kvm.c b/mm/kvm.c
new file mode 100644
index 000000000000..8945f6a311b9
--- /dev/null
+++ b/mm/kvm.c
@@ -0,0 +1,25 @@
+#include <linux/mman.h>
+
+int sysctl_kvm_madv_instant_free;
+
+int kvm_ret_mem_advice = MADV_DONTNEED;
+EXPORT_SYMBOL_GPL(kvm_ret_mem_advice);
+
+int kvm_madv_instant_free_sysctl_handler(struct ctl_table *table, int write,
+ void __user *buffer, size_t *length, loff_t *ppos)
+{
+ int ret;
+
+ ret = proc_dointvec(table, write, buffer, length, ppos);
+ if (ret)
+ return ret;
+
+#ifdef MADV_FREE
+ if (sysctl_kvm_madv_instant_free > 0)
+ kvm_ret_mem_advice = MADV_FREE;
+ else
+ kvm_ret_mem_advice = MADV_DONTNEED;
+#endif
+
+ return 0;
+}
--
2.12.1
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