[PATCH 1/3] mm/vmalloc: Move draining areas out of caller context

[Uladzislau Rezki (Sony)]

A caller initiates the drain procces from its context once the
drain threshold is reached or passed. There are at least two
drawbacks of doing so:

a) a caller can be a high-prio or RT task. In that case it can
   stuck in doing the actual drain of all lazily freed areas.
   This is not optimal because such tasks usually are latency
   sensitive where the control should be returned back as soon
   as possible in order to drive such workloads in time. See
   96e2db456135 ("mm/vmalloc: rework the drain logic")

b) It is not safe to call vfree() during holding a spinlock due
   to the vmap_purge_lock mutex. The was a report about this from
   Zeal Robot <zealci@xxxxxxxxxx> here:
   https://lore.kernel.org/all/20211222081026.484058-1-chi.minghao@xxxxxxxxxx

Moving the drain to the separate work context addresses those
issues.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
---
 mm/vmalloc.c | 35 ++++++++++++++++++++++-------------
 1 file changed, 22 insertions(+), 13 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index bdc7222f87d4..ed0f9eaa61a9 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -793,6 +793,9 @@ RB_DECLARE_CALLBACKS_MAX(static, free_vmap_area_rb_augment_cb,
 static void purge_vmap_area_lazy(void);
 static BLOCKING_NOTIFIER_HEAD(vmap_notify_list);
 static unsigned long lazy_max_pages(void);
+static void drain_vmap_area(struct work_struct *work);
+static DECLARE_WORK(drain_vmap_area_work, drain_vmap_area);
+static atomic_t drain_vmap_area_work_in_progress;
 
 static atomic_long_t nr_vmalloc_pages;
 
@@ -1719,18 +1722,6 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
 	return true;
 }
 
-/*
- * Kick off a purge of the outstanding lazy areas. Don't bother if somebody
- * is already purging.
- */
-static void try_purge_vmap_area_lazy(void)
-{
-	if (mutex_trylock(&vmap_purge_lock)) {
-		__purge_vmap_area_lazy(ULONG_MAX, 0);
-		mutex_unlock(&vmap_purge_lock);
-	}
-}
-
 /*
  * Kick off a purge of the outstanding lazy areas.
  */
@@ -1742,6 +1733,23 @@ static void purge_vmap_area_lazy(void)
 	mutex_unlock(&vmap_purge_lock);
 }
 
+static void drain_vmap_area(struct work_struct *work)
+{
+	unsigned long nr_lazy;
+
+	do {
+		mutex_lock(&vmap_purge_lock);
+		__purge_vmap_area_lazy(ULONG_MAX, 0);
+		mutex_unlock(&vmap_purge_lock);
+
+		/* Recheck if further work is required. */
+		nr_lazy = atomic_long_read(&vmap_lazy_nr);
+	} while (nr_lazy > lazy_max_pages());
+
+	/* We are done at this point. */
+	atomic_set(&drain_vmap_area_work_in_progress, 0);
+}
+
 /*
  * Free a vmap area, caller ensuring that the area has been unmapped
  * and flush_cache_vunmap had been called for the correct range
@@ -1768,7 +1776,8 @@ static void free_vmap_area_noflush(struct vmap_area *va)
 
 	/* After this point, we may free va at any time */
 	if (unlikely(nr_lazy > lazy_max_pages()))
-		try_purge_vmap_area_lazy();
+		if (!atomic_xchg(&drain_vmap_area_work_in_progress, 1))
+			schedule_work(&drain_vmap_area_work);
 }
 
 /*
-- 
2.30.2