Re: [PATCH] mm/slub: Move the stackdepot related allocation out of IRQ-off section.

[Vlastimil Babka <vbabka@suse.cz>]

On 6/7/22 17:20, Sebastian Andrzej Siewior wrote:
> The set_track() invocation in free_debug_processing() is invoked with
> acquired slab_lock(). The lock disables interrupts on PREEMPT_RT and
> this forbids to allocate memory which is done in stack_depot_save().
> 
> Split set_track() into two parts: set_track_prepare() which allocate
> memory and set_track_update() which only performs the assignment of the
> trace data structure. Use set_track_prepare() before disabling
> interrupts.
> 
> Fixes: 5cf909c553e9e ("mm/slub: use stackdepot to save stack trace in objects")
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Thanks!

What about calling set_track_update() from set_track() so the assignments
are not duplicated, like this?

----8<----
From c3fbd9cb11043c69f1073f438edd40d267f46cef Mon Sep 17 00:00:00 2001
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Tue, 7 Jun 2022 17:20:10 +0200
Subject: [PATCH] mm/slub: Move the stackdepot related allocation out of
 IRQ-off section.

The set_track() invocation in free_debug_processing() is invoked with
acquired slab_lock(). The lock disables interrupts on PREEMPT_RT and
this forbids to allocate memory which is done in stack_depot_save().

Split set_track() into two parts: set_track_prepare() which allocate
memory and set_track_update() which only performs the assignment of the
trace data structure. Use set_track_prepare() before disabling
interrupts.

[ vbabka@suse.cz: make set_track() call set_track_update() instead of
  open-coded assignments ]

Fixes: 5cf909c553e9e ("mm/slub: use stackdepot to save stack trace in objects")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Link: https://lore.kernel.org/r/Yp9sqoUi4fVa5ExF@linutronix.de
---
 mm/slub.c | 41 ++++++++++++++++++++++++++++++++++-------
 1 file changed, 34 insertions(+), 7 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index e5535020e0fd..f3b1e19b81f2 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -726,25 +726,48 @@ static struct track *get_track(struct kmem_cache *s, void *object,
 	return kasan_reset_tag(p + alloc);
 }
 
-static void noinline set_track(struct kmem_cache *s, void *object,
-			enum track_item alloc, unsigned long addr)
-{
-	struct track *p = get_track(s, object, alloc);
-
 #ifdef CONFIG_STACKDEPOT
+static noinline depot_stack_handle_t set_track_prepare(void)
+{
+	depot_stack_handle_t handle;
 	unsigned long entries[TRACK_ADDRS_COUNT];
 	unsigned int nr_entries;
 
 	nr_entries = stack_trace_save(entries, ARRAY_SIZE(entries), 3);
-	p->handle = stack_depot_save(entries, nr_entries, GFP_NOWAIT);
+	handle = stack_depot_save(entries, nr_entries, GFP_NOWAIT);
+
+	return handle;
+}
+#else
+static inline depot_stack_handle_t set_track_prepare(void)
+{
+	return 0;
+}
 #endif
 
+static void set_track_update(struct kmem_cache *s, void *object,
+			     enum track_item alloc, unsigned long addr,
+			     depot_stack_handle_t handle)
+{
+	struct track *p = get_track(s, object, alloc);
+
+#ifdef CONFIG_STACKDEPOT
+	p->handle = handle;
+#endif
 	p->addr = addr;
 	p->cpu = smp_processor_id();
 	p->pid = current->pid;
 	p->when = jiffies;
 }
 
+static __always_inline void set_track(struct kmem_cache *s, void *object,
+				      enum track_item alloc, unsigned long addr)
+{
+	depot_stack_handle_t handle = set_track_prepare();
+
+	set_track_update(s, object, alloc, addr, handle);
+}
+
 static void init_tracking(struct kmem_cache *s, void *object)
 {
 	struct track *p;
@@ -1373,6 +1396,10 @@ static noinline int free_debug_processing(
 	int cnt = 0;
 	unsigned long flags, flags2;
 	int ret = 0;
+	depot_stack_handle_t handle = 0;
+
+	if (s->flags & SLAB_STORE_USER)
+		handle = set_track_prepare();
 
 	spin_lock_irqsave(&n->list_lock, flags);
 	slab_lock(slab, &flags2);
@@ -1391,7 +1418,7 @@ static noinline int free_debug_processing(
 	}
 
 	if (s->flags & SLAB_STORE_USER)
-		set_track(s, object, TRACK_FREE, addr);
+		set_track_update(s, object, TRACK_FREE, addr, handle);
 	trace(s, slab, object, 0);
 	/* Freepointer not overwritten by init_object(), SLAB_POISON moved it */
 	init_object(s, object, SLUB_RED_INACTIVE);
-- 
2.36.1