Re: [PATCH v4 2/8] mm/memory_hotplug: Don't access uninitialized memmaps in shrink_zone_span()

[Aneesh Kumar K.V]

David Hildenbrand <david@xxxxxxxxxx> writes:

> Let's limit shrinking to !ZONE_DEVICE so we can fix the current code. We
> should never try to touch the memmap of offline sections where we could
> have uninitialized memmaps and could trigger BUGs when calling
> page_to_nid() on poisoned pages.
>
> Stopping to shrink the ZONE_DEVICE is fine as set_zone_contiguous() cannot
> deal with ZONE_DEVICE either way. The zones will always be !contiguous
> already and zone shrinking is therefore of limited use.

Can you add more details w.r.t ZONE_DEVICE?

>
> Before commit d0dc12e86b31 ("mm/memory_hotplug: optimize memory
> hotplug"), the memmap was initialized with 0 and the node with the
> right value. So the zone might be wrong but not garbage. After that
> commit, both the zone and the node will be garbage when touching
> uninitialized memmaps.
>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Oscar Salvador <osalvador@xxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Fixes: d0dc12e86b31 ("mm/memory_hotplug: optimize memory hotplug")
> Reported-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  mm/memory_hotplug.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index ddba8d786e4a..e0d1f6a9dfeb 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -331,7 +331,7 @@ static unsigned long find_smallest_section_pfn(int nid, struct zone *zone,
>  				     unsigned long end_pfn)
>  {
>  	for (; start_pfn < end_pfn; start_pfn += PAGES_PER_SUBSECTION) {
> -		if (unlikely(!pfn_valid(start_pfn)))
> +		if (unlikely(!pfn_to_online_page(start_pfn)))
>  			continue;
>  
>  		if (unlikely(pfn_to_nid(start_pfn) != nid))
> @@ -356,7 +356,7 @@ static unsigned long find_biggest_section_pfn(int nid, struct zone *zone,
>  	/* pfn is the end pfn of a memory section. */
>  	pfn = end_pfn - 1;
>  	for (; pfn >= start_pfn; pfn -= PAGES_PER_SUBSECTION) {
> -		if (unlikely(!pfn_valid(pfn)))
> +		if (unlikely(!pfn_to_online_page(pfn)))
>  			continue;
>  
>  		if (unlikely(pfn_to_nid(pfn) != nid))
> @@ -415,7 +415,7 @@ static void shrink_zone_span(struct zone *zone, unsigned long start_pfn,
>  	 */
>  	pfn = zone_start_pfn;
>  	for (; pfn < zone_end_pfn; pfn += PAGES_PER_SUBSECTION) {
> -		if (unlikely(!pfn_valid(pfn)))
> +		if (unlikely(!pfn_to_online_page(pfn)))
>  			continue;
>  
>  		if (page_zone(pfn_to_page(pfn)) != zone)
> @@ -463,6 +463,14 @@ static void __remove_zone(struct zone *zone, unsigned long start_pfn,
>  	struct pglist_data *pgdat = zone->zone_pgdat;
>  	unsigned long flags;
>  
> +	/*
> +	 * Zone shrinking code cannot properly deal with ZONE_DEVICE. So
> +	 * we will not try to shrink the zones - which is okay as
> +	 * set_zone_contiguous() cannot deal with ZONE_DEVICE either way.
> +	 */
> +	if (zone_idx(zone) == ZONE_DEVICE)
> +		return;

Can you describe here what is special about ZONE_DEVICE?


>  	pgdat_resize_lock(zone->zone_pgdat, &flags);
>  	shrink_zone_span(zone, start_pfn, start_pfn + nr_pages);
>  	update_pgdat_span(pgdat);
> -- 
> 2.21.0