linux-kernel - Re: [PATCH v4 2/2] mm: avoid slub allocation while holding list

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Date:   Sat, 9 Nov 2019 20:52:29 +0000 (UTC)
From:   Christopher Lameter <cl@...ux.com>
To:     Yu Zhao <yuzhao@...gle.com>
cc:     Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v4 2/2] mm: avoid slub allocation while holding
 list_lock

On Fri, 8 Nov 2019, Yu Zhao wrote:

> If we are already under list_lock, don't call kmalloc(). Otherwise we
> will run into deadlock because kmalloc() also tries to grab the same
> lock.

How did this happen? The kmalloc needs to be always done before the
list_lock is taken.

> Fixing the problem by using a static bitmap instead.
>
>   WARNING: possible recursive locking detected
>   --------------------------------------------
>   mount-encrypted/4921 is trying to acquire lock:
>   (&(&n->list_lock)->rlock){-.-.}, at: ___slab_alloc+0x104/0x437
>
>   but task is already holding lock:
>   (&(&n->list_lock)->rlock){-.-.}, at: __kmem_cache_shutdown+0x81/0x3cb
>
>   other info that might help us debug this:
>    Possible unsafe locking scenario:
>
>          CPU0
>          ----
>     lock(&(&n->list_lock)->rlock);
>     lock(&(&n->list_lock)->rlock);
>
>    *** DEADLOCK ***


Ahh. list_slab_objects() in shutdown?

There is a much easier fix for this:



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

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 <cl@...ux.com>

Index: linux/mm/slub.c
===================================================================
--- linux.orig/mm/slub.c	2019-10-15 13:54:57.032655296 +0000
+++ linux/mm/slub.c	2019-11-09 20:43:52.374187381 +0000
@@ -3690,14 +3690,11 @@ 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);

@@ -3723,6 +3720,10 @@ static void free_partial(struct kmem_cac
 {
 	LIST_HEAD(discard);
 	struct page *page, *h;
+	unsigned long *map = bitmap_alloc(oo_objects(s->max), GFP_KERNEL);
+
+	if (!map)
+		return;

 	BUG_ON(irqs_disabled());
 	spin_lock_irq(&n->list_lock);
@@ -3732,7 +3733,8 @@ 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);