Re: [patch] mm, page_alloc: move_freepages should not examine struct page of reserved memory

[Vlastimil Babka]

On 8/13/19 5:37 AM, David Rientjes wrote:
> After commit 907ec5fca3dc ("mm: zero remaining unavailable struct pages"),
> struct page of reserved memory is zeroed.  This causes page->flags to be 0
> and fixes issues related to reading /proc/kpageflags, for example, of
> reserved memory.
> 
> The VM_BUG_ON() in move_freepages_block(), however, assumes that
> page_zone() is meaningful even for reserved memory.  That assumption is no
> longer true after the aforementioned commit.

How comes that move_freepages_block() gets called on reserved memory in
the first place?

> There's no reason why move_freepages_block() should be testing the
> legitimacy of page_zone() for reserved memory; its scope is limited only
> to pages on the zone's freelist.
> 
> Note that pfn_valid() can be true for reserved memory: there is a backing
> struct page.  The check for page_to_nid(page) is also buggy but reserved
> memory normally only appears on node 0 so the zeroing doesn't affect this.
> 
> Move the debug checks to after verifying PageBuddy is true.  This isolates
> the scope of the checks to only be for buddy pages which are on the zone's
> freelist which move_freepages_block() is operating on.  In this case, an
> incorrect node or zone is a bug worthy of being warned about (and the
> examination of struct page is acceptable bcause this memory is not
> reserved).
> 
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
> ---
>  mm/page_alloc.c | 19 ++++---------------
>  1 file changed, 4 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2238,27 +2238,12 @@ static int move_freepages(struct zone *zone,
>  	unsigned int order;
>  	int pages_moved = 0;
>  
> -#ifndef CONFIG_HOLES_IN_ZONE
> -	/*
> -	 * page_zone is not safe to call in this context when
> -	 * CONFIG_HOLES_IN_ZONE is set. This bug check is probably redundant
> -	 * anyway as we check zone boundaries in move_freepages_block().
> -	 * Remove at a later date when no bug reports exist related to
> -	 * grouping pages by mobility
> -	 */
> -	VM_BUG_ON(pfn_valid(page_to_pfn(start_page)) &&
> -	          pfn_valid(page_to_pfn(end_page)) &&
> -	          page_zone(start_page) != page_zone(end_page));
> -#endif
>  	for (page = start_page; page <= end_page;) {
>  		if (!pfn_valid_within(page_to_pfn(page))) {
>  			page++;
>  			continue;
>  		}
>  
> -		/* Make sure we are not inadvertently changing nodes */
> -		VM_BUG_ON_PAGE(page_to_nid(page) != zone_to_nid(zone), page);
> -
>  		if (!PageBuddy(page)) {
>  			/*
>  			 * We assume that pages that could be isolated for
> @@ -2273,6 +2258,10 @@ static int move_freepages(struct zone *zone,
>  			continue;
>  		}
>  
> +		/* Make sure we are not inadvertently changing nodes */
> +		VM_BUG_ON_PAGE(page_to_nid(page) != zone_to_nid(zone), page);
> +		VM_BUG_ON_PAGE(page_zone(page) != zone, page);

The later check implies the former check, so if it's to stay, the first
one could be removed and comment adjusted s/nodes/zones/

> +
>  		order = page_order(page);
>  		move_to_free_area(page, &zone->free_area[order], migratetype);
>  		page += 1 << order;
>