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Subject: Re: [PATCH v1 1/2] powerpc/pseries/hotplug-memory: stop checking
 is_mem_section_removable()
To:     Michal Hocko <mhocko@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linuxppc-dev@lists.ozlabs.org,
        Nathan Fontenot <nfont@linux.vnet.ibm.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oscar Salvador <osalvador@suse.de>,
        Baoquan He <bhe@redhat.com>,
        Wei Yang <richard.weiyang@gmail.com>
References: <20200407135416.24093-1-david@redhat.com>
 <20200407135416.24093-2-david@redhat.com> <87sghdjf1y.fsf@mpe.ellerman.id.au>
 <20200409075927.GC18386@dhcp22.suse.cz>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <0a0ed3ed-f792-f8f7-07f4-cacc2b565a95@redhat.com>
Date:   Thu, 9 Apr 2020 10:12:20 +0200
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On 09.04.20 09:59, Michal Hocko wrote:
> On Thu 09-04-20 17:26:01, Michael Ellerman wrote:
>> David Hildenbrand <david@redhat.com> writes:
>>
>>> In commit 53cdc1cb29e8 ("drivers/base/memory.c: indicate all memory
>>> blocks as removable"), the user space interface to compute whether a memory
>>> block can be offlined (exposed via
>>> /sys/devices/system/memory/memoryX/removable) has effectively been
>>> deprecated. We want to remove the leftovers of the kernel implementation.
>>>
>>> When offlining a memory block (mm/memory_hotplug.c:__offline_pages()),
>>> we'll start by:
>>> 1. Testing if it contains any holes, and reject if so
>>> 2. Testing if pages belong to different zones, and reject if so
>>> 3. Isolating the page range, checking if it contains any unmovable pages
>>>
>>> Using is_mem_section_removable() before trying to offline is not only racy,
>>> it can easily result in false positives/negatives. Let's stop manually
>>> checking is_mem_section_removable(), and let device_offline() handle it
>>> completely instead. We can remove the racy is_mem_section_removable()
>>> implementation next.
>>>
>>> We now take more locks (e.g., memory hotplug lock when offlining and the
>>> zone lock when isolating), but maybe we should optimize that
>>> implementation instead if this ever becomes a real problem (after all,
>>> memory unplug is already an expensive operation). We started using
>>> is_mem_section_removable() in commit 51925fb3c5c9 ("powerpc/pseries:
>>> Implement memory hotplug remove in the kernel"), with the initial
>>> hotremove support of lmbs.
>>
>> It's also not very pretty in dmesg.
>>
>> Before:
>>
>>   pseries-hotplug-mem: Attempting to hot-add 10 LMB(s)
>>   pseries-hotplug-mem: Memory hot-add failed, removing any added LMBs
>>   dlpar: Could not handle DLPAR request "memory add count 10"
> 
> Yeah, there is more output but isn't that useful? Or put it differently
> what is the actual problem from having those messages in the kernel log?
> 
> From the below you can clearly tell that there are kernel allocations
> which prevent hot remove from happening.
> 
> If the overall size of the debugging output is a concern then we can
> think of a way to reduce it. E.g. once you have a couple of pages
> reported then all others from the same block are likely not interesting
> much.
> 

IIRC, we only report one page per block already. (and stop, as we
detected something unmovable)

-- 
Thanks,

David / dhildenb