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

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Message-ID: <0c649fb2-cbcc-48a8-8d6d-9d7f8a58858d@intel.com>
Date: Wed, 28 Jan 2026 22:11:47 +0800
From: "Li, Tianyou" <tianyou.li@...el.com>
To: Mike Rapoport <rppt@...nel.org>
CC: David Hildenbrand <david@...nel.org>, Oscar Salvador <osalvador@...e.de>,
	Wei Yang <richard.weiyang@...il.com>, Michal Hocko <mhocko@...e.com>,
	<linux-mm@...ck.org>, Yong Hu <yong.hu@...el.com>, Nanhai Zou
	<nanhai.zou@...el.com>, Yuan Liu <yuan1.liu@...el.com>, Tim Chen
	<tim.c.chen@...ux.intel.com>, Qiuxu Zhuo <qiuxu.zhuo@...el.com>, Yu C Chen
	<yu.c.chen@...el.com>, Pan Deng <pan.deng@...el.com>, Chen Zhang
	<zhangchen.kidd@...com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 3/3] mm/memory hotplug/unplug: Optimize
 zone->contiguous update when changes pfn range


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