The patch titled Subject: mm: fix balloon_page_movable() page->flags check has been added to the -mm tree. Its filename is mm-introduce-a-common-interface-for-balloon-pages-mobility-mm-fix-balloon_page_movable-page-flags-check.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/SubmitChecklist when testing your code *** The -mm tree is included into linux-next and is updated there every 3-4 working days ------------------------------------------------------ From: Rafael Aquini <aquini@xxxxxxxxxx> Subject: mm: fix balloon_page_movable() page->flags check Fix the following crash by fixing and enhancing the way page->flags are tested to identify a ballooned page. BUG: unable to handle kernel NULL pointer dereference at 0000000000000194 IP: [<ffffffff8122b354>] isolate_migratepages_range+0x344/0x7b0 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. Signed-off-by: Rafael Aquini <aquini@xxxxxxxxxx> Reported-by: Sasha Levin <levinsasha928@xxxxxxxxx> Cc: Mel Gorman <mel@xxxxxxxxx> Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx> Cc: "Michael S. Tsirkin" <mst@xxxxxxxxxx> Cc: Rik van Riel <riel@xxxxxxxxxx> Cc: Andi Kleen <andi@xxxxxxxxxxxxxx> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> Cc: Minchan Kim <minchan@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- include/linux/balloon_compaction.h | 21 +++++++++++++++++++-- 1 file changed, 19 insertions(+), 2 deletions(-) diff -puN include/linux/balloon_compaction.h~mm-introduce-a-common-interface-for-balloon-pages-mobility-mm-fix-balloon_page_movable-page-flags-check include/linux/balloon_compaction.h --- a/include/linux/balloon_compaction.h~mm-introduce-a-common-interface-for-balloon-pages-mobility-mm-fix-balloon_page_movable-page-flags-check +++ a/include/linux/balloon_compaction.h @@ -107,6 +107,23 @@ static inline void balloon_mapping_free( } /* + * __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( * 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; _ Patches currently in -mm which might be from aquini@xxxxxxxxxx are mm-adjust-address_space_operationsmigratepage-return-code.patch mm-redefine-address_spaceassoc_mapping.patch mm-introduce-a-common-interface-for-balloon-pages-mobility.patch mm-introduce-a-common-interface-for-balloon-pages-mobility-mm-fix-balloon_page_movable-page-flags-check.patch mm-introduce-compaction-and-migration-for-ballooned-pages.patch virtio_balloon-introduce-migration-primitives-to-balloon-pages.patch virtio_balloon-introduce-migration-primitives-to-balloon-pages-fix-fix.patch virtio_balloon-introduce-migration-primitives-to-balloon-pages-fix-fix-fix.patch mm-introduce-putback_movable_pages.patch mm-add-vm-event-counters-for-balloon-pages-compaction.patch -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html