Re: [PATCH] mm: fix balloon_page_movable() page->flags check

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 27 Nov 2012 21:31:10 -0200
Rafael Aquini <aquini@redhat.com> wrote:

> This patch fixes the following crash by fixing and enhancing the way 
> page->flags are tested to identify a ballooned page.
> 
> ---8<---
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000194
> IP: [<ffffffff8122b354>] isolate_migratepages_range+0x344/0x7b0
> --->8---
> 
> The NULL pointer deref was taking place because balloon_page_movable()
> page->flags tests were incomplete and we ended up 
> inadvertently poking at private pages.
> 
> ....
>
>  /*
> + * __balloon_page_flags - helper to perform balloon @page ->flags tests.
> + *
> + * As balloon pages are got from Buddy, and we do not play with page->flags
> + * at driver level (exception made when we get the page lock for compaction),
> + * therefore we can safely identify a ballooned page by checking if the
> + * NR_PAGEFLAGS rightmost bits from the page->flags are all cleared.
> + * This approach also helps on skipping ballooned pages that are locked for
> + * compaction or release, thus mitigating their racy check at
> + * balloon_page_movable()
> + */
> +#define BALLOON_PAGE_FLAGS_MASK       ((1UL << NR_PAGEFLAGS) - 1)

hm, this seems a bit fragile.

What's actually going on here?  You're assuming that a page fresh from
buddy has all flags 0..NR_PAGEFLAGS cleared?

That may be true, I'm unsure.  Please take a look at
PAGE_FLAGS_CHECK_AT_FREE and PAGE_FLAGS_CHECK_AT_PREP and work out why
the heck these aren't the same thing!

Either way around, doing

	#define BALLOON_PAGE_FLAGS_MASK PAGE_FLAGS_CHECK_AT_PREP

seems rather more maintainable.

> +static inline bool __balloon_page_flags(struct page *page)
> +{
> +	return page->flags & BALLOON_PAGE_FLAGS_MASK ? false : true;

We have a neater way of doing the scalar-to-boolean conversion:

	return !(page->flags & BALLOON_PAGE_FLAGS_MASK);

> +}
> +
> +/*
>   * __is_movable_balloon_page - helper to perform @page mapping->flags tests
>   */
>  static inline bool __is_movable_balloon_page(struct page *page)
> @@ -135,8 +152,8 @@ static inline bool balloon_page_movable(struct page *page)
>  	 * Before dereferencing and testing mapping->flags, lets make sure
>  	 * this is not a page that uses ->mapping in a different way
>  	 */
> -	if (!PageSlab(page) && !PageSwapCache(page) && !PageAnon(page) &&
> -	    !page_mapped(page))
> +	if (__balloon_page_flags(page) && !page_mapped(page) &&
> +	    page_count(page) == 1)
>  		return __is_movable_balloon_page(page);
>  
>  	return false;
>
> ...
>

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