Re: [PATCH] lib/scatterlist: Hook sg_kmalloc into kmemleak (v2)

[Catalin Marinas]

Jens,

On Tue, 2010-07-27 at 09:44 +0100, Chris Wilson wrote:
> kmemleak ignores page_alloc() and so believes the final sub-page
> allocation using the plain kmalloc is decoupled and lost. This leads to
> lots of false-positives with code that uses scatterlists.
> 
> The options seem to be either to tell kmemleak that the kmalloc is not
> leaked or to notify kmemleak of the page allocations. The danger of the
> first approach is that we may hide a real leak, so choose the latter
> approach (of which I am not sure of the downsides).
> 
> v2: Added comments on the suggestion of Catalin.
> 
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Jens Axboe <jaxboe@xxxxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>

Since you are the author of lib/scatterlist.c would you (or Tejun) mind
ack'ing this patch? Thanks.

> ---
>  lib/scatterlist.c |   21 ++++++++++++++++-----
>  1 files changed, 16 insertions(+), 5 deletions(-)
> 
> diff --git a/lib/scatterlist.c b/lib/scatterlist.c
> index 9afa25b..fc85552 100644
> --- a/lib/scatterlist.c
> +++ b/lib/scatterlist.c
> @@ -115,17 +115,28 @@ EXPORT_SYMBOL(sg_init_one);
>   */
>  static struct scatterlist *sg_kmalloc(unsigned int nents, gfp_t gfp_mask)
>  {
> -       if (nents == SG_MAX_SINGLE_ALLOC)
> -               return (struct scatterlist *) __get_free_page(gfp_mask);
> -       else
> +       if (nents == SG_MAX_SINGLE_ALLOC) {
> +               /* kmemleak doesn't track page allocations as they are not
> +                * commonly used (in a raw form) for kernel data structures.
> +                * As we chain together a list of pages and then a normal
> +                * kmalloc (tracked by kmemleak), in order to for that last
> +                * allocation not to become decoupled (and thus a
> +                * false-positive) we need to inform kmemleak of all the
> +                * intermediate allocations.
> +                */
> +               void *ptr = (void *) __get_free_page(gfp_mask);
> +               kmemleak_alloc(ptr, PAGE_SIZE, 1, gfp_mask);
> +               return ptr;
> +       } else
>                 return kmalloc(nents * sizeof(struct scatterlist), gfp_mask);
>  }
> 
>  static void sg_kfree(struct scatterlist *sg, unsigned int nents)
>  {
> -       if (nents == SG_MAX_SINGLE_ALLOC)
> +       if (nents == SG_MAX_SINGLE_ALLOC) {
> +               kmemleak_free(sg);
>                 free_page((unsigned long) sg);
> -       else
> +       } else
>                 kfree(sg);
>  }

-- 
Catalin

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