linux-kernel - Re: [PATCH 1/3] mm/slob: Drop usage of page->private for storing page-sized allocations

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Date:	Thu, 18 Oct 2012 15:46:49 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Ezequiel Garcia <elezegarcia@...il.com>
cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Tim Bird <tim.bird@...sony.com>,
	Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>
Subject: Re: [PATCH 1/3] mm/slob: Drop usage of page->private for storing
 page-sized allocations

On Thu, 18 Oct 2012, Ezequiel Garcia wrote:

> This field was being used to store size allocation so it could be
> retrieved by ksize(). However, it is a bad practice to not mark a page
> as a slab page and then use fields for special purposes.
> There is no need to store the allocated size and
> ksize() can simply return PAGE_SIZE << compound_order(page).
> 
> Cc: Pekka Penberg <penberg@...nel.org>

Is Pekka Penberg the long distant cousin of Pekka Enberg? :)  You should 
probably cc the author of slob, Matt Mackall <mpm@...enic.com>, on slob 
patches.

> Acked-by: Christoph Lameter <cl@...ux.com>
> Signed-off-by: Ezequiel Garcia <elezegarcia@...il.com>
> ---
>  mm/slob.c |   24 ++++++++++--------------
>  1 files changed, 10 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/slob.c b/mm/slob.c
> index a08e468..06a5ec7 100644
> --- a/mm/slob.c
> +++ b/mm/slob.c
> @@ -28,9 +28,8 @@
>   * from kmalloc are prepended with a 4-byte header with the kmalloc size.
>   * If kmalloc is asked for objects of PAGE_SIZE or larger, it calls
>   * alloc_pages() directly, allocating compound pages so the page order
> - * does not have to be separately tracked, and also stores the exact
> - * allocation size in page->private so that it can be used to accurately
> - * provide ksize(). These objects are detected in kfree() because slob_page()
> + * does not have to be separately tracked.
> + * These objects are detected in kfree() because PageSlab()
>   * is false for them.
>   *
>   * SLAB is emulated on top of SLOB by simply calling constructors and
> @@ -455,11 +454,6 @@ __do_kmalloc_node(size_t size, gfp_t gfp, int node, unsigned long caller)
>  		if (likely(order))
>  			gfp |= __GFP_COMP;
>  		ret = slob_new_pages(gfp, order, node);
> -		if (ret) {
> -			struct page *page;
> -			page = virt_to_page(ret);
> -			page->private = size;
> -		}
>  
>  		trace_kmalloc_node(caller, ret,
>  				   size, PAGE_SIZE << order, gfp, node);
> @@ -514,18 +508,20 @@ EXPORT_SYMBOL(kfree);
>  size_t ksize(const void *block)
>  {
>  	struct page *sp;
> +	int align;
> +	unsigned int *m;
>  
>  	BUG_ON(!block);
>  	if (unlikely(block == ZERO_SIZE_PTR))
>  		return 0;
>  
>  	sp = virt_to_page(block);
> -	if (PageSlab(sp)) {
> -		int align = max(ARCH_KMALLOC_MINALIGN, ARCH_SLAB_MINALIGN);
> -		unsigned int *m = (unsigned int *)(block - align);
> -		return SLOB_UNITS(*m) * SLOB_UNIT;
> -	} else
> -		return sp->private;
> +	if (unlikely(!PageSlab(sp)))
> +		return PAGE_SIZE << compound_order(sp);
> +
> +	align = max(ARCH_KMALLOC_MINALIGN, ARCH_SLAB_MINALIGN);
> +	m = (unsigned int *)(block - align);
> +	return SLOB_UNITS(*m) * SLOB_UNIT;
>  }
>  EXPORT_SYMBOL(ksize);
>  
--
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