On Tue, 27 Nov 2012 21:31:10 -0200
Rafael Aquini <aquini@xxxxxxxxxx> 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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