Re: [PATCH] mm, slob: Drop usage of page->private for storing page-sized allocations

[Pekka Enberg <penberg@kernel.org>]

On Tue, Aug 14, 2012 at 11:03 PM, Ezequiel Garcia <elezegarcia@gmail.com> 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 Enberg <penberg@kernel.org>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Glauber Costa <glommer@parallels.com>
> Signed-off-by: Ezequiel Garcia <elezegarcia@gmail.com>

Looks good to me. Matt?

> ---
>  mm/slob.c |   23 ++++++++++-------------
>  1 files changed, 10 insertions(+), 13 deletions(-)
>
> diff --git a/mm/slob.c b/mm/slob.c
> index 686e98b..987da93 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
> @@ -450,7 +449,6 @@ void *__kmalloc_node(size_t size, gfp_t gfp, int node)
>                                    size, size + align, gfp, node);
>         } else {
>                 unsigned int order = get_order(size);
> -               struct page *page;
>
>                 if (likely(order))
>                         gfp |= __GFP_COMP;
> @@ -458,9 +456,6 @@ void *__kmalloc_node(size_t size, gfp_t gfp, int node)
>                 if (!ret)
>                         return NULL;
>
> -               page = virt_to_page(ret);
> -               page->private = size;
> -
>                 trace_kmalloc_node(_RET_IP_, ret,
>                                    size, PAGE_SIZE << order, gfp, node);
>         }
> @@ -494,18 +489,20 @@ 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);
>
> --
> 1.7.8.6
>
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