Re: [PATCHv3 4/5] mm: make compound_head() robust

[Jesper Dangaard Brouer]

On Wed, 19 Aug 2015 12:21:45 +0300
"Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:

> Hugh has pointed that compound_head() call can be unsafe in some
> context. There's one example:
> 
[...]

> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 0735bc0a351a..a4c4b7d07473 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h

[...]
> -/*
> - * If we access compound page synchronously such as access to
> - * allocated page, there is no need to handle tail flag race, so we can
> - * check tail flag directly without any synchronization primitive.
> - */
> -static inline struct page *compound_head_fast(struct page *page)
> -{
> -	if (unlikely(PageTail(page)))
> -		return page->first_page;
> -	return page;
> -}
> -
[...]

> @@ -548,13 +508,7 @@ static inline struct page *virt_to_head_page(const void *x)
>  {
>  	struct page *page = virt_to_page(x);
>  
> -	/*
> -	 * We don't need to worry about synchronization of tail flag
> -	 * when we call virt_to_head_page() since it is only called for
> -	 * already allocated page and this page won't be freed until
> -	 * this virt_to_head_page() is finished. So use _fast variant.
> -	 */
> -	return compound_head_fast(page);
> +	return compound_head(page);
>  }

I hope this does not slow down the SLAB/slub allocator?
(which calls virt_to_head_page() frequently)

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Sr. Network Kernel Developer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer
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