Re: [PATCH v8 3/3] mm/memory hotplug/unplug: Optimize zone->contiguous update when changes pfn range

[Li, Tianyou]

On 1/27/2026 3:10 PM, Mike Rapoport wrote:
> Hi,
>
> On Sat, Jan 24, 2026 at 08:43:51PM +0800, Li, Tianyou wrote:
> > On 1/22/2026 7:43 PM, Mike Rapoport wrote:
> >
> > > > +int online_memory_block_pages(unsigned long start_pfn, unsigned long nr_pages,
> > > > +			unsigned long nr_vmemmap_pages, struct zone *zone,
> > > > +			struct memory_group *group)
> > > >    {
> > > > +	const bool contiguous = zone->contiguous;
> > > > +	enum zone_contig_state new_contiguous_state;
> > > >    	int ret;
> > > > +	/*
> > > > +	 * Calculate the new zone contig state before move_pfn_range_to_zone()
> > > > +	 * sets the zone temporarily to non-contiguous.
> > > > +	 */
> > > > +	new_contiguous_state = zone_contig_state_after_growing(zone, start_pfn,
> > > > +							       nr_pages);
> > > > +
> > > >    	if (nr_vmemmap_pages) {
> > > >    		ret = mhp_init_memmap_on_memory(start_pfn, nr_vmemmap_pages, zone);
> > > >    		if (ret)
> > > > -			return ret;
> > > > +			goto restore_zone_contig;
> > > But zone_contig_state_after_growing() does not change zone->contiguous. Why
> > > do we need to save and restore it?
> > Move_pfn_range_to_zone() will clear the zone contiguous state and it was
> > invoked by online_pages(). If error occurs after
> > move_pfn_range_to_zone() called like in online_pages(), I think we'd better
> > to restore the original value if previous zone contiguous state is true.
>   
> But after move_pfn_range_to_zone() the added pages are still offline, so I
> think the zone remains contiguous and the call to
> clear_zone_contiguous(zone) should not be there.

Since move_pfn_range_to_zone() may change the zone contiguous state, 
clear_zone_contiguous() should be invoked. If we did not 
clear_zone_contiguous() properly, especially keep the zone contiguous 
state as true but actually the zone is non contiguous after resize the 
zone, the code path rely on the contiguous state potentially may fail?

I am not sure if we need to handle the clear_zone_contiguous() in or out 
of move_pfn_range_to_zone() in this patch series.

> BTW, as we have set_zone_contiguous(ZONE_CONTIG_NO) I think we can use it
> instead if clear_zone_contiguous() and remove the latter.
>   

Yes we can. I am hesitate to do so because clear_zone_contiguous() has a 
name that explain itself. The pair of clear/set seems a pattern should 
be preserved? I am OK to change the code, let's hear from other comments 
if feasible? Thanks.


Regards,

Tianyou