linux-kernel - Re: [PATCH 1/8] badpage: simplify page

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Date:	Mon, 1 Dec 2008 08:47:22 -0600 (CST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Hugh Dickins <hugh@...itas.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Russ Anderson <rja@....com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Dave Jones <davej@...hat.com>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/8] badpage: simplify page_alloc flag check+clear

On Mon, 1 Dec 2008, Hugh Dickins wrote:

>  /*
>   * Flags checked when a page is freed.  Pages being freed should not have
>   * these flags set.  It they are, there is a problem.
>   */
> -#define PAGE_FLAGS_CHECK_AT_FREE (PAGE_FLAGS | 1 << PG_reserved)
> +#define PAGE_FLAGS_CHECK_AT_FREE \
> +	(1 << PG_lru   | 1 << PG_private   | 1 << PG_locked | \
> +	 1 << PG_buddy | 1 << PG_writeback | 1 << PG_reserved | \
> +	 1 << PG_slab  | 1 << PG_swapcache | 1 << PG_active | \
> +	 __PG_UNEVICTABLE | __PG_MLOCKED)

Rename this to PAGE_FLAGS_CLEAR_WHEN_FREE?

> + * Pages being prepped should not have any flags set.  It they are set,
> + * there has been a kernel bug or struct page corruption.
>   */
> -#define PAGE_FLAGS_CHECK_AT_PREP (PAGE_FLAGS | \
> -		1 << PG_reserved | 1 << PG_dirty | 1 << PG_swapbacked)
> +#define PAGE_FLAGS_CHECK_AT_PREP	((1 << NR_PAGEFLAGS) - 1)

These are all the bits. Can we get rid of this definition?

>  	/*
>  	 * For now, we report if PG_reserved was found set, but do not
>  	 * clear it, and do not free the page.  But we shall soon need
>  	 * to do more, for when the ZERO_PAGE count wraps negative.
>  	 */
> -	return PageReserved(page);
> +	if (PageReserved(page))
> +		return 1;
> +	if (page->flags & PAGE_FLAGS_CHECK_AT_PREP)
> +		page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
> +	return 0;

The name PAGE_FLAGS_CHECK_AT_PREP is strange. We clear these flags without
message. This is equal to

page->flags &=~PAGE_FLAGS_CHECK_AT_PREP;

You can drop the if...

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