Re: [PATCH v1 1/3] mm/shuffle: don't move pages between zones and don't read garbage memmaps

[Michal Hocko <mhocko@kernel.org>]

On Tue 16-06-20 13:52:11, David Hildenbrand wrote:
> Especially with memory hotplug, we can have offline sections (with a
> garbage memmap) and overlapping zones. We have to make sure to only
> touch initialized memmaps (online sections managed by the buddy) and that
> the zone matches, to not move pages between zones.
> 
> To test if this can actually happen, I added a simple
> 	BUG_ON(page_zone(page_i) != page_zone(page_j));
> right before the swap. When hotplugging a 256M DIMM to a 4G x86-64 VM and
> onlining the first memory block "online_movable" and the second memory
> block "online_kernel", it will trigger the BUG, as both zones (NORMAL
> and MOVABLE) overlap.
> 
> This might result in all kinds of weird situations (e.g., double
> allocations, list corruptions, unmovable allocations ending up in the
> movable zone).
> 
> Fixes: e900a918b098 ("mm: shuffle initial free memory to improve memory-side-cache utilization")
> Cc: stable@vger.kernel.org # v5.2+
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Huang Ying <ying.huang@intel.com>
> Cc: Wei Yang <richard.weiyang@gmail.com>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!

> ---
>  mm/shuffle.c | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/shuffle.c b/mm/shuffle.c
> index 44406d9977c77..dd13ab851b3ee 100644
> --- a/mm/shuffle.c
> +++ b/mm/shuffle.c
> @@ -58,25 +58,25 @@ module_param_call(shuffle, shuffle_store, shuffle_show, &shuffle_param, 0400);
>   * For two pages to be swapped in the shuffle, they must be free (on a
>   * 'free_area' lru), have the same order, and have the same migratetype.
>   */
> -static struct page * __meminit shuffle_valid_page(unsigned long pfn, int order)
> +static struct page * __meminit shuffle_valid_page(struct zone *zone,
> +						  unsigned long pfn, int order)
>  {
> -	struct page *page;
> +	struct page *page = pfn_to_online_page(pfn);
>  
>  	/*
>  	 * Given we're dealing with randomly selected pfns in a zone we
>  	 * need to ask questions like...
>  	 */
>  
> -	/* ...is the pfn even in the memmap? */
> -	if (!pfn_valid_within(pfn))
> +	/* ... is the page managed by the buddy? */
> +	if (!page)
>  		return NULL;
>  
> -	/* ...is the pfn in a present section or a hole? */
> -	if (!pfn_in_present_section(pfn))
> +	/* ... is the page assigned to the same zone? */
> +	if (page_zone(page) != zone)
>  		return NULL;
>  
>  	/* ...is the page free and currently on a free_area list? */
> -	page = pfn_to_page(pfn);
>  	if (!PageBuddy(page))
>  		return NULL;
>  
> @@ -123,7 +123,7 @@ void __meminit __shuffle_zone(struct zone *z)
>  		 * page_j randomly selected in the span @zone_start_pfn to
>  		 * @spanned_pages.
>  		 */
> -		page_i = shuffle_valid_page(i, order);
> +		page_i = shuffle_valid_page(z, i, order);
>  		if (!page_i)
>  			continue;
>  
> @@ -137,7 +137,7 @@ void __meminit __shuffle_zone(struct zone *z)
>  			j = z->zone_start_pfn +
>  				ALIGN_DOWN(get_random_long() % z->spanned_pages,
>  						order_pages);
> -			page_j = shuffle_valid_page(j, order);
> +			page_j = shuffle_valid_page(z, j, order);
>  			if (page_j && page_j != page_i)
>  				break;
>  		}
> -- 
> 2.26.2

-- 
Michal Hocko
SUSE Labs