Re: [PATCH 4/6] mm: page_alloc: fix move_freepages_block() range error

[Vlastimil Babka <vbabka@suse.cz>]

On 9/11/23 21:41, 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>

Note below:

> ---
>  mm/page_alloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index f6f658c3d394..5bbe5f3be5ad 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1652,7 +1652,7 @@ int move_freepages_block(struct zone *zone, struct page *page,
>  
>  	/* Do not cross zone boundaries */
>  	if (!zone_spans_pfn(zone, start_pfn))
> -		start_pfn = pfn;
> +		start_pfn = zone->zone_start_pfn;
>  	if (!zone_spans_pfn(zone, end_pfn))
>  		return 0;

Culdn't we also adjust end_pfn to zone_end_pfn() so we don't just ignore the
last half-pageblock for no good reason? (or am I missing any?)
Also would stop treating end_pfn as inclusive here and in move_freepages(),
it's rather uncommon.

>