[PATCH v3 1/3] mm/vmalloc: remove preempt_disable/enable when do preloading

["Uladzislau Rezki (Sony)" <urezki@gmail.com>]

Some background. The preemption was disabled before to guarantee
that a preloaded object is available for a CPU, it was stored for.

The aim was to not allocate in atomic context when spinlock
is taken later, for regular vmap allocations. But that approach
conflicts with CONFIG_PREEMPT_RT philosophy. It means that
calling spin_lock() with disabled preemption is forbidden
in the CONFIG_PREEMPT_RT kernel.

Therefore, get rid of preempt_disable() and preempt_enable() when
the preload is done for splitting purpose. As a result we do not
guarantee now that a CPU is preloaded, instead we minimize the
case when it is not, with this change.

For example i run the special test case that follows the preload
pattern and path. 20 "unbind" threads run it and each does
1000000 allocations. Only 3.5 times among 1000000 a CPU was
not preloaded. So it can happen but the number is negligible.

V2 - > V3:
    - update the commit message

V1 -> V2:
  - move __this_cpu_cmpxchg check when spin_lock is taken,
    as proposed by Andrew Morton
  - add more explanation in regard of preloading
  - adjust and move some comments

Fixes: 82dd23e84be3 ("mm/vmalloc.c: preload a CPU with one object for split purpose")
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
 mm/vmalloc.c | 37 ++++++++++++++++++++-----------------
 1 file changed, 20 insertions(+), 17 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index e92ff5f7dd8b..b7b443bfdd92 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1078,31 +1078,34 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
 
 retry:
 	/*
-	 * Preload this CPU with one extra vmap_area object to ensure
-	 * that we have it available when fit type of free area is
-	 * NE_FIT_TYPE.
+	 * Preload this CPU with one extra vmap_area object. It is used
+	 * when fit type of free area is NE_FIT_TYPE. Please note, it
+	 * does not guarantee that an allocation occurs on a CPU that
+	 * is preloaded, instead we minimize the case when it is not.
+	 * It can happen because of cpu migration, because there is a
+	 * race until the below spinlock is taken.
 	 *
 	 * The preload is done in non-atomic context, thus it allows us
 	 * to use more permissive allocation masks to be more stable under
-	 * low memory condition and high memory pressure.
+	 * low memory condition and high memory pressure. In rare case,
+	 * if not preloaded, GFP_NOWAIT is used.
 	 *
-	 * Even if it fails we do not really care about that. Just proceed
-	 * as it is. "overflow" path will refill the cache we allocate from.
+	 * Set "pva" to NULL here, because of "retry" path.
 	 */
-	preempt_disable();
-	if (!__this_cpu_read(ne_fit_preload_node)) {
-		preempt_enable();
-		pva = kmem_cache_alloc_node(vmap_area_cachep, GFP_KERNEL, node);
-		preempt_disable();
+	pva = NULL;
 
-		if (__this_cpu_cmpxchg(ne_fit_preload_node, NULL, pva)) {
-			if (pva)
-				kmem_cache_free(vmap_area_cachep, pva);
-		}
-	}
+	if (!this_cpu_read(ne_fit_preload_node))
+		/*
+		 * Even if it fails we do not really care about that.
+		 * Just proceed as it is. If needed "overflow" path
+		 * will refill the cache we allocate from.
+		 */
+		pva = kmem_cache_alloc_node(vmap_area_cachep, GFP_KERNEL, node);
 
 	spin_lock(&vmap_area_lock);
-	preempt_enable();
+
+	if (pva && __this_cpu_cmpxchg(ne_fit_preload_node, NULL, pva))
+		kmem_cache_free(vmap_area_cachep, pva);
 
 	/*
 	 * If an allocation fails, the "vend" address is
-- 
2.20.1