Re: [PATCH 1/2] mm, slob: Prevent false positive trace upon allocation failure

Ezequiel Garcia
2012-08-15T12:34:51Z

Pekka,

I'd like to bring your attention to this patch, and make a little question.

On Tue, Aug 14, 2012 at 11:38 AM, Ezequiel Garcia <elezegarcia@gmail.com> wrote:
> This patch changes the __kmalloc_node() logic to return NULL
> if alloc_pages() fails to return valid pages.
> This is done to avoid to trace a false positive kmalloc event.
>
> 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>
> ---
> mm/slob.c | 11 ++++++-----
> 1 files changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/mm/slob.c b/mm/slob.c
> index 45d4ca7..686e98b 100644
> --- a/mm/slob.c
> +++ b/mm/slob.c
> @@ -450,15 +450,16 @@ 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;
> ret = slob_new_pages(gfp, order, node);
> - if (ret) {
> - struct page *page;
> - page = virt_to_page(ret);
> - page->private = size;
> - }
> + if (!ret)
> + return NULL;
> +
> + page = virt_to_page(ret);
> + page->private = size;
>
> trace_kmalloc_node(_RET_IP_, ret,
> size, PAGE_SIZE << order, gfp, node);

As you can see this patch prevents to trace a kmem event if the allocation
fails.

I'm still unsure about tracing or not this ones, and I'm considering tracing
failures, perhaps with return=0 and allocated size=0.

In this case, it would be nice to have SLxB all do the same.
Right now, this is not the case.

You can see how slob::kmem_cache_alloc_node traces independently
of the allocation succeeding.
I have no problem trying a fix for this, but I don't now how to trace
this cases.

Although it is a corner case, I think it's important to define a clear
and consistent
behaviour to make tracing reliable.

Thanks,
Ezequiel.