Re: [PATCH 05/10] mm: page_alloc: fix move_freepages_block() range error

[Vlastimil Babka <vbabka@suse.cz>]

On 3/20/24 7:02 PM, Johannes Weiner wrote:
> When a block is partially outside the zone of the cursor page, the
> function cuts the range to the pivot page instead of the zone
> start. This can leave large parts of the block behind, which
> encourages incompatible page mixing down the line (ask for one type,
> get another), and thus long-term fragmentation.
> 
> This triggers reliably on the first block in the DMA zone, whose
> start_pfn is 1. The block is stolen, but everything before the pivot
> page (which was often hundreds of pages) is left on the old list.
> 
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Reviewed-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  mm/page_alloc.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index a1376a6fe7e4..7373329763e6 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1645,9 +1645,15 @@ int move_freepages_block(struct zone *zone, struct page *page,
>  	start_pfn = pageblock_start_pfn(pfn);
>  	end_pfn = pageblock_end_pfn(pfn) - 1;
>  
> -	/* Do not cross zone boundaries */
> +	/*
> +	 * The caller only has the lock for @zone, don't touch ranges
> +	 * that straddle into other zones. While we could move part of
> +	 * the range that's inside the zone, this call is usually
> +	 * accompanied by other operations such as migratetype updates
> +	 * which also should be locked.
> +	 */
>  	if (!zone_spans_pfn(zone, start_pfn))
> -		start_pfn = pfn;
> +		return 0;
>  	if (!zone_spans_pfn(zone, end_pfn))
>  		return 0;
>