[RFC PATCH 5/7] lazy tlb: introduce lazy mm refcount helper functions

Nicholas Piggin
2020-07-10T01:57:36Z

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

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
arch/powerpc/kernel/smp.c | 2 +-
arch/powerpc/mm/book3s64/radix_tlb.c | 4 ++--
fs/exec.c | 2 +-
include/linux/sched/mm.h | 17 +++++++++++++++++
kernel/cpu.c | 2 +-
kernel/exit.c | 2 +-
kernel/kthread.c | 11 +++++++----
kernel/sched/core.c | 13 +++++++------
8 files changed, 37 insertions(+), 16 deletions(-)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 73199470c265..ad95812d2a3f 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -1253,7 +1253,7 @@ void start_secondary(void *unused)
unsigned int cpu = smp_processor_id();
struct cpumask *(*sibling_mask)(int) = cpu_sibling_mask;

- mmgrab(&init_mm);
+ mmgrab(&init_mm); /* XXX: where is the mmput for this? */
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 b5cc9b23cf02..52730629b3eb 100644
--- a/arch/powerpc/mm/book3s64/radix_tlb.c
+++ b/arch/powerpc/mm/book3s64/radix_tlb.c
@@ -652,10 +652,10 @@ static void do_exit_flush_lazy_tlb(void *arg)
* Must be a kernel thread because sender is single-threaded.
*/
BUG_ON(current->mm);
- mmgrab(&init_mm);
+ mmgrab_lazy_tlb(&init_mm);
switch_mm(mm, &init_mm, current);
current->active_mm = &init_mm;
- mmdrop(mm);
+ mmdrop_lazy_tlb(mm);
}
_tlbiel_pid(pid, RIC_FLUSH_ALL);
}
diff --git a/fs/exec.c b/fs/exec.c
index e2ab71e88293..3a01b2751ea9 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1119,7 +1119,7 @@ static int exec_mmap(struct mm_struct *mm)
mmput(old_mm);
} else {
exit_lazy_tlb(active_mm, tsk);
- mmdrop(active_mm);
+ mmdrop_lazy_tlb(active_mm);
}
return 0;
}
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 9b026264b445..110d4ad21de6 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -50,6 +50,23 @@ static inline void mmdrop(struct mm_struct *mm)

void mmdrop(struct mm_struct *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);
+}
+
+static inline void mmdrop_lazy_tlb_smp_mb(struct mm_struct *mm)
+{
+ /* This depends on mmdrop providing a full smp_mb() */
+ mmdrop(mm);
+}
+
/*
* This has to be called after a get_task_mm()/mmget_not_zero()
* followed by taking the mmap_lock for writing before modifying the
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 134688d79589..ff9fcbc4e76b 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -578,7 +578,7 @@ 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 727150f28103..d535da9fd2f8 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -470,7 +470,7 @@ 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 6f93c649aa97..a7133cc2ddaf 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -1238,12 +1238,12 @@ 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);
active_mm = tsk->active_mm;
- if (active_mm != mm) {
- mmgrab(mm);
+ if (active_mm != mm)
tsk->active_mm = mm;
- }
tsk->mm = mm;
switch_mm(active_mm, mm, tsk);
task_unlock(tsk);
@@ -1253,7 +1253,7 @@ void kthread_use_mm(struct mm_struct *mm)

exit_lazy_tlb(active_mm, tsk);
if (active_mm != mm)
- mmdrop(active_mm);
+ mmdrop_lazy_tlb(active_mm);

to_kthread(tsk)->oldfs = get_fs();
set_fs(USER_DS);
@@ -1276,9 +1276,12 @@ void kthread_unuse_mm(struct mm_struct *mm)
task_lock(tsk);
sync_mm_rss(mm);
tsk->mm = NULL;
+ mmgrab_lazy_tlb(mm);
/* active_mm is still 'mm' */
enter_lazy_tlb(mm, tsk);
task_unlock(tsk);
+
+ mmdrop(mm);
}
EXPORT_SYMBOL_GPL(kthread_unuse_mm);

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 31e22c79826c..d19f2f517f6c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3302,10 +3302,11 @@ static struct rq *finish_task_switch(struct task_struct *prev)
* schedule between user->kernel->user threads without passing though
* switch_mm(). Membarrier requires a full barrier after storing to
* rq->curr, before returning to userspace, for
- * {PRIVATE,GLOBAL}_EXPEDITED. This is implicitly provided by mmdrop().
+ * {PRIVATE,GLOBAL}_EXPEDITED. This is implicitly provided by
+ * mmdrop_lazy_tlb_smp_mb().
*/
if (mm)
- mmdrop(mm);
+ mmdrop_lazy_tlb_smp_mb(mm);

if (unlikely(prev_state == TASK_DEAD)) {
if (prev->sched_class->task_dead)
@@ -3410,9 +3411,9 @@ 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
@@ -3420,7 +3421,7 @@ 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
@@ -3438,7 +3439,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
if (!prev->mm) { // from kernel
exit_lazy_tlb(prev->active_mm, next);

- /* will mmdrop() in finish_task_switch(). */
+ /* will mmdrop_lazy_tlb() in finish_task_switch(). */
rq->prev_mm = prev->active_mm;
prev->active_mm = NULL;
}
--
2.23.0