Re: [PATCH v4 2/2] mm: avoid slub allocation while holding list_lock

[Yu Zhao]

On Sat, Nov 09, 2019 at 08:52:29PM +0000, Christopher Lameter wrote:
> 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@xxxxxxxxx>
> 
> 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;

What would happen if we are trying to allocate from the slab that is
being shut down? And shouldn't the allocation be conditional (i.e.,
only when CONFIG_SLUB_DEBUG=y)?