linux-kernel - Re: [PATCH v2 8/8] mm/memory_hotplug: allow to specify a default online

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Date:   Tue, 17 Mar 2020 12:05:50 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linuxppc-dev@...ts.ozlabs.org, linux-hyperv@...r.kernel.org,
        Wei Yang <richard.weiyang@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Baoquan He <bhe@...hat.com>
Subject: Re: [PATCH v2 8/8] mm/memory_hotplug: allow to specify a default
 online_type

On 17.03.20 12:01, Michal Hocko wrote:
> On Tue 17-03-20 11:49:42, David Hildenbrand wrote:
>> For now, distributions implement advanced udev rules to essentially
>> - Don't online any hotplugged memory (s390x)
>> - Online all memory to ZONE_NORMAL (e.g., most virt environments like
>>   hyperv)
>> - Online all memory to ZONE_MOVABLE in case the zone imbalance is taken
>>   care of (e.g., bare metal, special virt environments)
>>
>> In summary: All memory is usually onlined the same way, however, the
>> kernel always has to ask user space to come up with the same answer.
>> E.g., Hyper-V always waits for a memory block to get onlined before
>> continuing, otherwise it might end up adding memory faster than
>> hotplugging it, which can result in strange OOM situations. This waiting
>> slows down adding of a bigger amount of memory.
>>
>> Let's allow to specify a default online_type, not just "online" and
>> "offline". This allows distributions to configure the default online_type
>> when booting up and be done with it.
>>
>> We can now specify "offline", "online", "online_movable" and
>> "online_kernel" via
>> - "memhp_default_state=" on the kernel cmdline
>> - /sys/devices/system/memory/auto_online_blocks
>> just like we are able to specify for a single memory block via
>> /sys/devices/system/memory/memoryX/state
>>
>> Reviewed-by: Wei Yang <richard.weiyang@...il.com>
>> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: Michal Hocko <mhocko@...nel.org>
>> Cc: Oscar Salvador <osalvador@...e.de>
>> Cc: "Rafael J. Wysocki" <rafael@...nel.org>
>> Cc: Baoquan He <bhe@...hat.com>
>> Cc: Wei Yang <richard.weiyang@...il.com>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
> 
> As I've said earlier and several times already, I really dislike this
> interface. But it is fact that this patch doesn't make it any worse.
> Quite contrary, so feel free to add
> Acked-by: Michal Hocko <mhocko@...e.com>

Thanks Michal!

-- 
Thanks,

David / dhildenb