linux-kernel - Re: [PATCH] slab: fix oops when reading /proc/slab

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Date:	Tue, 17 Jun 2014 11:29:33 +0400
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	Christoph Lameter <cl@...ux.com>,
	David Rientjes <rientjes@...gle.com>,
	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
	<stable@...r.kernel.org>
Subject: Re: [PATCH] slab: fix oops when reading /proc/slab_allocators

Hi,

On Tue, Jun 17, 2014 at 10:09:52AM +0900, Joonsoo Kim wrote:
[...]
> To fix the problem, I introduces object status buffer on each slab.
> With this, we can track object status precisely, so slab leak detector
> would not access active object and no kernel oops would occur.
> Memory overhead caused by this fix is only imposed to
> CONFIG_DEBUG_SLAB_LEAK which is mainly used for debugging, so memory
> overhead isn't big problem.
[...]
>  
> +static size_t calculate_freelist_size(int nr_objs, size_t align)
> +{
> +	size_t freelist_size;
> +
> +	freelist_size = nr_objs * sizeof(freelist_idx_t);
> +	if (IS_ENABLED(CONFIG_DEBUG_SLAB_LEAK))
> +		freelist_size += nr_objs * sizeof(char);
> +
> +	if (align)
> +		freelist_size = ALIGN(freelist_size, align);
> +
> +	return freelist_size;
> +}
> +
>  static int calculate_nr_objs(size_t slab_size, size_t buffer_size,
>  				size_t idx_size, size_t align)
>  {
>  	int nr_objs;
> +	size_t remained_size;
>  	size_t freelist_size;
> +	int extra_space = 0;
>  
> +	if (IS_ENABLED(CONFIG_DEBUG_SLAB_LEAK))
> +		extra_space = sizeof(char);
>  	/*
>  	 * Ignore padding for the initial guess. The padding
>  	 * is at most @align-1 bytes, and @buffer_size is at
> @@ -590,14 +641,15 @@ static int calculate_nr_objs(size_t slab_size, size_t buffer_size,
>  	 * into the memory allocation when taking the padding
>  	 * into account.
>  	 */
> -	nr_objs = slab_size / (buffer_size + idx_size);
> +	nr_objs = slab_size / (buffer_size + idx_size + extra_space);

There is one more function that wants to know how much space per object
is spent for management. It's calculate_slab_order():

	if (flags & CFLGS_OFF_SLAB) {
		/*
		 * Max number of objs-per-slab for caches which
		 * use off-slab slabs. Needed to avoid a possible
		 * looping condition in cache_grow().
		 */
		offslab_limit = size;
		offslab_limit /= sizeof(freelist_idx_t);

		if (num > offslab_limit)
			break;
	}

May be, we should update it too?

Thanks.
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