[PATCH v3 1/4] lazy tlb: introduce lazy mm refcount helper functions

Nicholas Piggin
2021-06-01T06:23:18Z

Add explicit _lazy_tlb annotated functions for lazy mm refcounting.
This makes lazy mm references more obvious, and allows explicit
refcounting to be removed if it is not used.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
arch/arm/mach-rpc/ecard.c | 2 +-
arch/powerpc/kernel/smp.c | 2 +-
arch/powerpc/mm/book3s64/radix_tlb.c | 4 ++--
fs/exec.c | 4 ++--
include/linux/sched/mm.h | 11 +++++++++++
kernel/cpu.c | 2 +-
kernel/exit.c | 2 +-
kernel/kthread.c | 11 +++++++----
kernel/sched/core.c | 15 ++++++++-------
9 files changed, 34 insertions(+), 19 deletions(-)

diff --git a/arch/arm/mach-rpc/ecard.c b/arch/arm/mach-rpc/ecard.c
index 827b50f1c73e..1b4a41aad793 100644
--- a/arch/arm/mach-rpc/ecard.c
+++ b/arch/arm/mach-rpc/ecard.c
@@ -253,7 +253,7 @@ static int ecard_init_mm(void)
current->mm = mm;
current->active_mm = mm;
activate_mm(active_mm, mm);
- mmdrop(active_mm);
+ mmdrop_lazy_tlb(active_mm);
ecard_init_pgtables(mm);
return 0;
}
diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 2e05c783440a..fb0bdfc67366 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -1541,7 +1541,7 @@ void start_secondary(void *unused)
{
unsigned int cpu = raw_smp_processor_id();

- mmgrab(&init_mm);
+ mmgrab_lazy_tlb(&init_mm);
current->active_mm = &init_mm;

smp_store_cpu_info(cpu);
diff --git a/arch/powerpc/mm/book3s64/radix_tlb.c b/arch/powerpc/mm/book3s64/radix_tlb.c
index 409e61210789..2962082787c0 100644
--- a/arch/powerpc/mm/book3s64/radix_tlb.c
+++ b/arch/powerpc/mm/book3s64/radix_tlb.c
@@ -663,10 +663,10 @@ void exit_lazy_flush_tlb(struct mm_struct *mm, bool always_flush)
if (current->active_mm == mm) {
WARN_ON_ONCE(current->mm != NULL);
/* Is a kernel thread and is using mm as the lazy tlb */
- mmgrab(&init_mm);
+ mmgrab_lazy_tlb(&init_mm);
current->active_mm = &init_mm;
switch_mm_irqs_off(mm, &init_mm, current);
- mmdrop(mm);
+ mmdrop_lazy_tlb(mm);
}

/*
diff --git a/fs/exec.c b/fs/exec.c
index 18594f11c31f..ca0f8b1af23a 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1029,9 +1029,9 @@ static int exec_mmap(struct mm_struct *mm)
setmax_mm_hiwater_rss(&tsk->signal->maxrss, old_mm);
mm_update_next_owner(old_mm);
mmput(old_mm);
- return 0;
+ } else {
+ mmdrop_lazy_tlb(active_mm);
}
- mmdrop(active_mm);
return 0;
}

diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index e24b1fe348e3..bfd1baca5266 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -49,6 +49,17 @@ static inline void mmdrop(struct mm_struct *mm)
__mmdrop(mm);
}

+/* Helpers for lazy TLB mm refcounting */
+static inline void mmgrab_lazy_tlb(struct mm_struct *mm)
+{
+ mmgrab(mm);
+}
+
+static inline void mmdrop_lazy_tlb(struct mm_struct *mm)
+{
+ mmdrop(mm);
+}
+
/**
* mmget() - Pin the address space associated with a &struct mm_struct.
* @mm: The address space to pin.
diff --git a/kernel/cpu.c b/kernel/cpu.c
index e538518556f4..e87a89824e6c 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -602,7 +602,7 @@ static int finish_cpu(unsigned int cpu)
*/
if (mm != &init_mm)
idle->active_mm = &init_mm;
- mmdrop(mm);
+ mmdrop_lazy_tlb(mm);
return 0;
}

diff --git a/kernel/exit.c b/kernel/exit.c
index fd1c04193e18..8e87ec5f6be2 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -476,7 +476,7 @@ static void exit_mm(void)
__set_current_state(TASK_RUNNING);
mmap_read_lock(mm);
}
- mmgrab(mm);
+ mmgrab_lazy_tlb(mm);
BUG_ON(mm != current->active_mm);
/* more a memory barrier than a real lock */
task_lock(current);
diff --git a/kernel/kthread.c b/kernel/kthread.c
index fe3f2a40d61e..b70e28431a01 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -1314,14 +1314,14 @@ void kthread_use_mm(struct mm_struct *mm)
WARN_ON_ONCE(!(tsk->flags & PF_KTHREAD));
WARN_ON_ONCE(tsk->mm);

+ mmgrab(mm);
+
task_lock(tsk);
/* Hold off tlb flush IPIs while switching mm's */
local_irq_disable();
active_mm = tsk->active_mm;
- if (active_mm != mm) {
- mmgrab(mm);
+ if (active_mm != mm)
tsk->active_mm = mm;
- }
tsk->mm = mm;
membarrier_update_current_mm(mm);
switch_mm_irqs_off(active_mm, mm, tsk);
@@ -1341,7 +1341,7 @@ void kthread_use_mm(struct mm_struct *mm)
* mmdrop(), or explicitly with smp_mb().
*/
if (active_mm != mm)
- mmdrop(active_mm);
+ mmdrop_lazy_tlb(active_mm);
else
smp_mb();

@@ -1375,10 +1375,13 @@ void kthread_unuse_mm(struct mm_struct *mm)
local_irq_disable();
tsk->mm = NULL;
membarrier_update_current_mm(NULL);
+ mmgrab_lazy_tlb(mm);
/* active_mm is still 'mm' */
enter_lazy_tlb(mm, tsk);
local_irq_enable();
task_unlock(tsk);
+
+ mmdrop(mm);
}
EXPORT_SYMBOL_GPL(kthread_unuse_mm);

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5226cc26a095..e359c76ea2e2 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4229,13 +4229,14 @@ static struct rq *finish_task_switch(struct task_struct *prev)
* rq->curr, before returning to userspace, so provide them here:
*
* - a full memory barrier for {PRIVATE,GLOBAL}_EXPEDITED, implicitly
- * provided by mmdrop(),
+ * provided by mmdrop_lazy_tlb(),
* - a sync_core for SYNC_CORE.
*/
if (mm) {
membarrier_mm_sync_core_before_usermode(mm);
- mmdrop(mm);
+ mmdrop_lazy_tlb(mm);
}
+
if (unlikely(prev_state == TASK_DEAD)) {
if (prev->sched_class->task_dead)
prev->sched_class->task_dead(prev);
@@ -4299,9 +4300,9 @@ context_switch(struct rq *rq, struct task_struct *prev,

/*
* kernel -> kernel lazy + transfer active
- * user -> kernel lazy + mmgrab() active
+ * user -> kernel lazy + mmgrab_lazy_tlb() active
*
- * kernel -> user switch + mmdrop() active
+ * kernel -> user switch + mmdrop_lazy_tlb() active
* user -> user switch
*/
if (!next->mm) { // to kernel
@@ -4309,7 +4310,7 @@ context_switch(struct rq *rq, struct task_struct *prev,

next->active_mm = prev->active_mm;
if (prev->mm) // from user
- mmgrab(prev->active_mm);
+ mmgrab_lazy_tlb(prev->active_mm);
else
prev->active_mm = NULL;
} else { // to user
@@ -4325,7 +4326,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
switch_mm_irqs_off(prev->active_mm, next->mm, next);

if (!prev->mm) { // from kernel
- /* will mmdrop() in finish_task_switch(). */
+ /* will mmdrop_lazy_tlb() in finish_task_switch(). */
rq->prev_mm = prev->active_mm;
prev->active_mm = NULL;
}
@@ -8239,7 +8240,7 @@ void __init sched_init(void)
/*
* The boot idle thread does lazy MMU switching as well:
*/
- mmgrab(&init_mm);
+ mmgrab_lazy_tlb(&init_mm);
enter_lazy_tlb(&init_mm, current);

/*
--
2.23.0