Re: [PATCH 2/2] mm: verify compound order when freeing a page

[David Cohen <david.a.cohen@linux.intel.com>]

Hi Yu,

On Tue, Oct 14, 2014 at 01:16:40PM -0700, Yu Zhao wrote:
> This allows us to easily catch the bug fixed in previous patch.

Is the word "previous" a good way to relate patches after merged?
Maybe you could either detail the bug here or be more verbose about the
patch you're referring.

> 
> Here we also verify whether a page is tail page or not -- tail
> pages are supposed to be freed along with their head, not by
> themselves.
> 
> Signed-off-by: Yu Zhao <yuzhao@google.com>
> ---
>  mm/page_alloc.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 736d8e1..2bcc770 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -750,6 +750,9 @@ static bool free_pages_prepare(struct page *page, unsigned int order)
>  	int i;
>  	int bad = 0;
>  
> +	VM_BUG_ON(PageTail(page));
> +	VM_BUG_ON(PageHead(page) && compound_order(page) != order);

It may be too severe. AFAIU we're not talking about a fatal error.
How about VM_WARN_ON()?

Br, David Cohen

> +
>  	trace_mm_page_free(page, order);
>  	kmemcheck_free_shadow(page, order);
>  
> -- 
> 2.1.0.rc2.206.gedb03e5
> 
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