linux-kernel - Re: [PATCH RFC v4 08/13] mm/memory_hotplug: Introduce offline_and_remove

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Date:   Tue, 25 Feb 2020 15:27:28 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        virtio-dev@...ts.oasis-open.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Oscar Salvador <osalvador@...e.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Wei Yang <richard.weiyang@...il.com>,
        Dan Williams <dan.j.williams@...el.com>, Qian Cai <cai@....pw>
Subject: Re: [PATCH RFC v4 08/13] mm/memory_hotplug: Introduce
 offline_and_remove_memory()

On 25.02.20 15:11, Michal Hocko wrote:
> On Thu 12-12-19 18:11:32, David Hildenbrand wrote:
>> virtio-mem wants to offline and remove a memory block once it unplugged
>> all subblocks (e.g., using alloc_contig_range()). Let's provide
>> an interface to do that from a driver. virtio-mem already supports to
>> offline partially unplugged memory blocks. Offlining a fully unplugged
>> memory block will not require to migrate any pages. All unplugged
>> subblocks are PageOffline() and have a reference count of 0 - so
>> offlining code will simply skip them.
>>
>> All we need an interface to trigger the "offlining" and the removing in a
>> single operation - to make sure the memory block cannot get onlined by
>> user space again before it gets removed.
> 
> Why does that matter? Is it really likely that the userspace would
> interfere? What would be the scenario?

I guess it's not that relevant after all (I think this comment dates
back to the times where we didn't have try_remove_memory() and could
actually BUG_ON() in remove_memory() if there would have been a race).
Can drop that part.

> 
> Or is still mostly about not requiring callers to open code this general
> patter?

>From kernel module context, I cannot get access to the actual memory
block device (find_memory_block()) and call the device_unregister().

Especially, also the device hotplug lock is not exported. So this is a
clean helper function to be used from kernel module context. (e.g., also
hyper-v showed interest for using that)

-- 
Thanks,

David / dhildenb