Re: [PATCH v2 for-4.4-stable] mm: migrate: don't rely on __PageMovable() of newpage after unlocking it

[David Hildenbrand]

On 01.02.19 14:43, David Hildenbrand wrote:
> This is the backport for 4.4-stable.
> 
> We had a race in the old balloon compaction code before commit b1123ea6d3b3
> ("mm: balloon: use general non-lru movable page feature") refactored it
> that became visible after backporting commit 195a8c43e93d
> ("virtio-balloon: deflate via a page list") without the refactoring.
> 
> The bug existed from commit d6d86c0a7f8d ("mm/balloon_compaction: redesign
> ballooned pages management") till commit b1123ea6d3b3 ("mm: balloon: use
> general non-lru movable page feature"). commit d6d86c0a7f8d
> ("mm/balloon_compaction: redesign ballooned pages management") was
> backported to 3.12, so the broken kernels are stable kernels [3.12 - 4.7].
> 
> There was a subtle race between dropping the page lock of the newpage
> in __unmap_and_move() and checking for
> __is_movable_balloon_page(newpage).
> 
> Just after dropping this page lock, virtio-balloon could go ahead and
> deflate the newpage, effectively dequeueing it and clearing PageBalloon,
> in turn making __is_movable_balloon_page(newpage) fail.
> 
> This resulted in dropping the reference of the newpage via
> putback_lru_page(newpage) instead of put_page(newpage), leading to
> page->lru getting modified and a !LRU page ending up in the LRU lists.
> With commit 195a8c43e93d ("virtio-balloon: deflate via a page list")
> backported, one would suddenly get corrupted lists in
> release_pages_balloon():
> - WARNING: CPU: 13 PID: 6586 at lib/list_debug.c:59 __list_del_entry+0xa1/0xd0
> - list_del corruption. prev->next should be ffffe253961090a0, but was dead000000000100
> 
> Nowadays this race is no longer possible, but it is hidden behind very
> ugly handling of __ClearPageMovable() and __PageMovable().
> 
> __ClearPageMovable() will not make __PageMovable() fail, only
> PageMovable(). So the new check (__PageMovable(newpage)) will still hold
> even after newpage was dequeued by virtio-balloon.
> 
> If anybody would ever change that special handling, the BUG would be
> introduced again. So instead, make it explicit and use the information
> of the original isolated page before migration.
> 
> This patch can be backported fairly easy to stable kernels (in contrast
> to the refactoring).
> 
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Vratislav Bendel <vbendel@xxxxxxxxxx>
> Cc: Rafael Aquini <aquini@xxxxxxxxxx>
> Cc: Konstantin Khlebnikov <k.khlebnikov@xxxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Sasha Levin <sashal@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # 3.12 - 4.7
> Fixes: d6d86c0a7f8d ("mm/balloon_compaction: redesign ballooned pages management")
> Reported-by: Vratislav Bendel <vbendel@xxxxxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> Acked-by: Rafael Aquini <aquini@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  mm/migrate.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index afedcfab60e2..3304c98f9a78 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -936,6 +936,7 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
>  	int rc = MIGRATEPAGE_SUCCESS;
>  	int *result = NULL;
>  	struct page *newpage;
> +	bool is_lru = !isolated_balloon_page(page);
>  
>  	newpage = get_new_page(page, private, &result);
>  	if (!newpage)
> @@ -984,10 +985,14 @@ out:
>  	 * If migration was not successful and there's a freeing callback, use
>  	 * it.  Otherwise, putback_lru_page() will drop the reference grabbed
>  	 * during isolation.
> +	 *
> +	 * Use the old state of the isolated source page to determine if we
> +	 * migrated a LRU page. newpage was already unlocked and possibly
> +	 * modified by its owner - don't rely on the page state.
>  	 */
>  	if (put_new_page)
>  		put_new_page(newpage, private);
> -	else if (unlikely(__is_movable_balloon_page(newpage))) {

And to be save, we should turn this into

 else if (rc == MIGRATEPAGE_SUCCESS && unlikely(!is_lru)) {

But will resend this either way as already mentioned to Greg.

> +	else if (unlikely(!is_lru)) {
>  		/* drop our reference, page already in the balloon */
>  		put_page(newpage);
>  	} else
> 


-- 

Thanks,

David / dhildenb