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

[Johannes Weiner]

On Wed, Sep 13, 2023 at 04:40:48PM +0200, Vlastimil Babka wrote:
> 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@xxxxxxxxxxx>
> 
> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>

Thanks!

> > @@ -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.

You raise a good point here and in the reply to 5/6. Let me reply to
the other email.