linux-kernel - Re: [PATCHv8] mm: slab: free kmem_cache

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Message-ID: <20160208095831.GB30053@esperanza>
Date:	Mon, 8 Feb 2016 12:58:31 +0300
From:	Vladimir Davydov <vdavydov@...tuozzo.com>
To:	Dmitry Safonov <dsafonov@...tuozzo.com>
CC:	<akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>, <0x7f454c46@...il.com>,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCHv8] mm: slab: free kmem_cache_node after destroy sysfs file

On Mon, Feb 08, 2016 at 12:31:47PM +0300, Dmitry Safonov wrote:
> With enabled slub_debug alloc_calls_show will try to track location and
> user of slab object on each online node, kmem_cache_node structure and
> cpu_cache/cpu_slub shouldn't be freed till there is the last reference
> to sysfs file.
> 
> Fixes the following panic:
> [43963.463055] BUG: unable to handle kernel
> [43963.463090] NULL pointer dereference at 0000000000000020
> [43963.463146] IP: [<ffffffff811c6959>] list_locations+0x169/0x4e0
> [43963.463185] PGD 257304067 PUD 438456067 PMD 0
> [43963.463220] Oops: 0000 [#1] SMP
> [43963.463850] CPU: 3 PID: 973074 Comm: cat ve: 0 Not tainted 3.10.0-229.7.2.ovz.9.30-00007-japdoll-dirty #2 9.30
> [43963.463913] Hardware name: DEPO Computers To Be Filled By O.E.M./H67DE3, BIOS L1.60c 07/14/2011
> [43963.463976] task: ffff88042a5dc5b0 ti: ffff88037f8d8000 task.ti: ffff88037f8d8000
> [43963.464036] RIP: 0010:[<ffffffff811c6959>]  [<ffffffff811c6959>] list_locations+0x169/0x4e0
> [43963.464725] Call Trace:
> [43963.464756]  [<ffffffff811c6d1d>] alloc_calls_show+0x1d/0x30
> [43963.464793]  [<ffffffff811c15ab>] slab_attr_show+0x1b/0x30
> [43963.464829]  [<ffffffff8125d27a>] sysfs_read_file+0x9a/0x1a0
> [43963.464865]  [<ffffffff811e3c6c>] vfs_read+0x9c/0x170
> [43963.464900]  [<ffffffff811e4798>] SyS_read+0x58/0xb0
> [43963.464936]  [<ffffffff81612d49>] system_call_fastpath+0x16/0x1b
> [43963.464970] Code: 5e 07 12 00 b9 00 04 00 00 3d 00 04 00 00 0f 4f c1 3d 00 04 00 00 89 45 b0 0f 84 c3 00 00 00 48 63 45 b0 49 8b 9c c4 f8 00 00 00 <48> 8b 43 20 48 85 c0 74 b6 48 89 df e8 46 37 44 00 48 8b 53 10
> [43963.465119] RIP  [<ffffffff811c6959>] list_locations+0x169/0x4e0
> [43963.465155]  RSP <ffff88037f8dbe28>
> [43963.465185] CR2: 0000000000000020
> 
> Separated __kmem_cache_release from __kmem_cache_shutdown which now
> called on slab_kmem_cache_release (after the last reference to sysfs
> file object has dropped).
> Reintroduced locking in free_partial as sysfs file might access cache's
> partial list after shutdowning - partiall revert of the
> commit 69cb8e6b7c2982 ("slub: free slabs without holding locks")
> 
> Cc: Christoph Lameter <cl@...ux.com>
> Cc: Pekka Enberg <penberg@...nel.org>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
> Suggested-by: Vladimir Davydov <vdavydov@...tuozzo.com>
> Signed-off-by: Dmitry Safonov <dsafonov@...tuozzo.com>

The patch looks good to me now, thanks.

Acked-by: Vladimir Davydov <vdavydov@...tuozzo.com>

Two minor nits below:

...
>  /*
>   * Attempt to free all partial slabs on a node.
> - * This is called from kmem_cache_close(). We must be the last thread
> - * using the cache and therefore we do not need to lock anymore.
> + * This is called from __kmem_cache_shutdown(). We must take list_lock
> + * because sysfs file might still access partial list after the shutdowning.
>   */
>  static void free_partial(struct kmem_cache *s, struct kmem_cache_node *n)
>  {
> +	unsigned long flags;
>  	struct page *page, *h;
>  
> +	spin_lock_irqsave(&n->list_lock, flags);

Nit: this function is never called with irqs disabled, so
spin_lock_irq() would be enough. We could add BUG_ON(irqs_disabled())
just to be sure.

>  	list_for_each_entry_safe(page, h, &n->partial, lru) {
>  		if (!page->inuse) {
>  			__remove_partial(n, page);

Nit: __remove_partial() (with leading underscores) was introduced solely
to avoid lockdep warning here (see 1e4dd9461fabf). Since we now take
list_lock, we can use remove_partial() (w/o underscores) and hence zap
__remove_partial().

Thanks,
Vladimir

>  			discard_slab(s, page);
>  		} else {
>  			list_slab_objects(s, page,
> -			"Objects remaining in %s on kmem_cache_close()");
> +			"Objects remaining in %s on __kmem_cache_shutdown()");
>  		}
>  	}
> +	spin_unlock_irqrestore(&n->list_lock, flags);
>  }