[PATCH] mm: drop hotplug lock from lru_add_drain_all

From: Michal Hocko
Date: Thu Nov 16 2017 - 07:05:55 EST

From: Michal Hocko <mhocko@xxxxxxxx>

Pulling cpu hotplug locks inside the mm core function like
lru_add_drain_all just asks for problems and the recent lockdep splat
[1] just proves this. While the usage in that particular case might
be wrong we should prevent from locking as lru_add_drain_all is used
at many places. It seems that this is not all that hard to achieve

We have done the same thing for drain_all_pages which is analogous by
a459eeb7b852 ("mm, page_alloc: do not depend on cpu hotplug locks inside
the allocator"). All we have to care about is to handle
- the work item might be executed on a different cpu in worker from
unbound pool so it doesn't run on pinned on the cpu

- we have to make sure that we do not race with page_alloc_cpu_dead
calling lru_add_drain_cpu

the first part is already handled because the worker calls lru_add_drain
which disables preemption when calling lru_add_drain_cpu on the local
cpu it is draining. The later is true because page_alloc_cpu_dead
is called on the controlling CPU after the hotplugged CPU vanished

[1] http://lkml.kernel.org/r/089e0825eec8955c1f055c83d476@xxxxxxxxxx

[add a cpu hotplug locking interaction as per tglx]
Acked-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
include/linux/swap.h | 1 -
mm/memory_hotplug.c | 2 +-
mm/swap.c | 16 ++++++++--------
3 files changed, 9 insertions(+), 10 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 84255b3da7c1..cfc200673e13 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -331,7 +331,6 @@ extern void mark_page_accessed(struct page *);
extern void lru_add_drain(void);
extern void lru_add_drain_cpu(int cpu);
extern void lru_add_drain_all(void);
-extern void lru_add_drain_all_cpuslocked(void);
extern void rotate_reclaimable_page(struct page *page);
extern void deactivate_file_page(struct page *page);
extern void mark_page_lazyfree(struct page *page);
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 832a042134f8..c9f6b418be79 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1641,7 +1641,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
goto failed_removal;

- lru_add_drain_all_cpuslocked();
+ lru_add_drain_all();

pfn = scan_movable_pages(start_pfn, end_pfn);
diff --git a/mm/swap.c b/mm/swap.c
index 381e0fe9efbf..1ab8122d2d0c 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -688,7 +688,14 @@ static void lru_add_drain_per_cpu(struct work_struct *dummy)

static DEFINE_PER_CPU(struct work_struct, lru_add_drain_work);

-void lru_add_drain_all_cpuslocked(void)
+ * Doesn't need any cpu hotplug locking because we do rely on per-cpu
+ * kworkers being shut down before our page_alloc_cpu_dead callback is
+ * executed on the offlined cpu.
+ * Calling this function with cpu hotplug locks held can actually lead
+ * to obscure indirect dependencies via WQ context.
+ */
+void lru_add_drain_all(void)
static DEFINE_MUTEX(lock);
static struct cpumask has_work;
@@ -724,13 +731,6 @@ void lru_add_drain_all_cpuslocked(void)

-void lru_add_drain_all(void)
- get_online_cpus();
- lru_add_drain_all_cpuslocked();
- put_online_cpus();
* release_pages - batched put_page()
* @pages: array of pages to release