linux-kernel - Re: [PATCH v1 0/2] mm/memory_hotplug: introduce and use PG_offline

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Message-ID: <7c508718-da88-4a37-b134-29aa89ee9899@redhat.com>
Date: Wed, 14 May 2025 19:46:35 +0200
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 virtualization@...ts.linux.dev, "Michael S. Tsirkin" <mst@...hat.com>,
 Jason Wang <jasowang@...hat.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
 Eugenio Pérez <eperezma@...hat.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Oscar Salvador
 <osalvador@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Brendan Jackman <jackmanb@...gle.com>, Johannes Weiner <hannes@...xchg.org>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>
Subject: Re: [PATCH v1 0/2] mm/memory_hotplug: introduce and use
 PG_offline_skippable

>>>> Next, the page is removed from its memory
>>>>> block. When will PG_offline_skippable be used? The second phase when
>>>>> the page is being removed from its memory block?
>>>>
>>>> PG_offline_skippable is used during memory offlining, while we look for any pages that are not PageBuddy (... or hwpoisoned ...), to migrate them off the memory so they get converted to PageBuddy.
>>>>
>>>> PageOffline + PageOfflineSkippable are checked on that phase, such that they don't require any migration.
>>>
>>> Hmm, if you just do not want to get PageOffline migrated, not setting it
>>> __PageMovable would work right? PageOffline + __PageMovable is used by
>>> ballooning, as these inflated pages can be migrated. PageOffline without
>>> __PageMovable should be virtio-mem. Am I missing any other user?
>>
>> Sure. Just imagine !CONFIG_BALLOON_COMPACTION.
>>
>> In summary, we have
>>
>> 1) Migratable PageOffline pages (balloon compaction)
>>
>> 2) Unmigratable PageOffline pages (e.g., XEN balloon, hyper-v balloon,
>>     memtrace, in the future likely some memory holes, ... )
>>
>> 3) Skippable PageOffline pages (virtio-mem)
> 
> Thank you for all the explanation. Now I understand how memory offline
> and memory hotunplug work and shall begin to check the patches. :)

Sure, if you think the doc or some comments could be updated, I'm happy 
to review such changes.

It's always very helpful to receive feedback from someone that's new to 
this code.

-- 
Cheers,

David / dhildenb