From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx164.postini.com [74.125.245.164]) by kanga.kvack.org (Postfix) with SMTP id 3AD146B00A9 for ; Tue, 27 Nov 2012 18:32:54 -0500 (EST) Date: Tue, 27 Nov 2012 21:32:44 -0200 From: Rafael Aquini Subject: Re: [PATCH] mm: fix balloon_page_movable() page->flags check Message-ID: <20121127233243.GB1812@t510.redhat.com> References: <20121127145708.c7173d0d.akpm@linux-foundation.org> <1ccb1c95a52185bcc6009761cb2829197e2737ea.1354058194.git.aquini@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1ccb1c95a52185bcc6009761cb2829197e2737ea.1354058194.git.aquini@redhat.com> Sender: owner-linux-mm@kvack.org List-ID: To: akpm@linux-foundation.org Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Sasha Levin , Mel Gorman , Rik van Riel On Tue, Nov 27, 2012 at 09:31:10PM -0200, Rafael Aquini 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: [] 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. > > Reported-by: Sasha Levin > Signed-off-by: Rafael Aquini > --- Here it is Andrew, sorry by the lagged reply Cheers! --Rafael > include/linux/balloon_compaction.h | 21 +++++++++++++++++++-- > 1 file changed, 19 insertions(+), 2 deletions(-) > > diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h > index 68893bc..634a19b 100644 > --- a/include/linux/balloon_compaction.h > +++ b/include/linux/balloon_compaction.h > @@ -107,6 +107,23 @@ static inline void balloon_mapping_free(struct address_space *balloon_mapping) > } > > /* > + * __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) > +static inline bool __balloon_page_flags(struct page *page) > +{ > + return page->flags & BALLOON_PAGE_FLAGS_MASK ? false : true; > +} > + > +/* > * __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; > -- > 1.7.11.7 > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org