Re: [patch 018/118] mm/slub.c: avoid slub allocation while holding list_lock

[Christopher Lameter <cl@xxxxxxxxx>]

On Thu, 30 Jan 2020, Andrew Morton wrote:

> From: Yu Zhao <yuzhao@xxxxxxxxxx>
> Subject: mm/slub.c: avoid slub allocation while holding list_lock
>
> If we are already under list_lock, don't call kmalloc().  Otherwise we
> will run into a deadlock because kmalloc() also tries to grab the same
> lock.

I provided another patch that moves the lock. Remember we agreed that this
here is a ugly hack.

[FIX] slub: Remove kmalloc under list_lock from list_slab_objects() V2

V1->V2 : Properly handle CONFIG_SLUB_DEBUG. Handle bitmap free correctly.

list_slab_objects() is called when a slab is destroyed and there are objects still left
to list the objects in the syslog. This is a pretty rare event.

And there it seems we take the list_lock and call kmalloc while holding that lock.

Perform the allocation in free_partial() before the list_lock is taken.

Fixes: bbd7d57bfe852d9788bae5fb171c7edb4021d8ac ("slub: Potential stack overflow")
Signed-off-by: Christoph Lameter

Index: linux/mm/slub.c
===================================================================
--- linux.orig/mm/slub.c	2019-10-15 13:54:57.032655296 +0000
+++ linux/mm/slub.c	2019-11-11 15:52:11.616397853 +0000
@@ -3690,14 +3690,15 @@ error:
 }

 static void list_slab_objects(struct kmem_cache *s, struct page *page,
-							const char *text)
+					const char *text, unsigned long *map)
 {
 #ifdef CONFIG_SLUB_DEBUG
 	void *addr = page_address(page);
 	void *p;
-	unsigned long *map = bitmap_zalloc(page->objects, GFP_ATOMIC);
+
 	if (!map)
 		return;
+
 	slab_err(s, page, text, s->name);
 	slab_lock(page);

@@ -3710,7 +3711,6 @@ static void list_slab_objects(struct kme
 		}
 	}
 	slab_unlock(page);
-	bitmap_free(map);
 #endif
 }

@@ -3723,6 +3723,11 @@ static void free_partial(struct kmem_cac
 {
 	LIST_HEAD(discard);
 	struct page *page, *h;
+	unsigned long *map = NULL;
+
+#ifdef CONFIG_SLUB_DEBUG
+	map = bitmap_alloc(oo_objects(s->max), GFP_KERNEL);
+#endif

 	BUG_ON(irqs_disabled());
 	spin_lock_irq(&n->list_lock);
@@ -3732,11 +3737,16 @@ static void free_partial(struct kmem_cac
 			list_add(&page->slab_list, &discard);
 		} else {
 			list_slab_objects(s, page,
-			"Objects remaining in %s on __kmem_cache_shutdown()");
+			"Objects remaining in %s on __kmem_cache_shutdown()",
+			map);
 		}
 	}
 	spin_unlock_irq(&n->list_lock);

+#ifdef CONFIG_SLUB_DEBUG
+	bitmap_free(map);
+#endif
+
 	list_for_each_entry_safe(page, h, &discard, slab_list)
 		discard_slab(s, page);
 }