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Subject: Re: [v5 0/3] "Hotremove" persistent memory
To:     Dan Williams <dan.j.williams@intel.com>,
        Pavel Tatashin <pasha.tatashin@soleen.com>
Cc:     "Verma, Vishal L" <vishal.l.verma@intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "jmorris@namei.org" <jmorris@namei.org>,
        "tiwai@suse.de" <tiwai@suse.de>,
        "sashal@kernel.org" <sashal@kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
        "bp@suse.de" <bp@suse.de>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>,
        "jglisse@redhat.com" <jglisse@redhat.com>,
        "zwisler@kernel.org" <zwisler@kernel.org>,
        "mhocko@suse.com" <mhocko@suse.com>,
        "Jiang, Dave" <dave.jiang@intel.com>,
        "bhelgaas@google.com" <bhelgaas@google.com>,
        "Busch, Keith" <keith.busch@intel.com>,
        "thomas.lendacky@amd.com" <thomas.lendacky@amd.com>,
        "Huang, Ying" <ying.huang@intel.com>,
        "Wu, Fengguang" <fengguang.wu@intel.com>,
        "baiyaowei@cmss.chinamobile.com" <baiyaowei@cmss.chinamobile.com>
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From:   David Hildenbrand <david@redhat.com>
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Date:   Thu, 16 May 2019 09:10:12 +0200
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On 16.05.19 02:42, Dan Williams wrote:
> On Wed, May 15, 2019 at 11:12 AM Pavel Tatashin
> <pasha.tatashin@soleen.com> wrote:
>>
>>> Hi Pavel,
>>>
>>> I am working on adding this sort of a workflow into a new daxctl command
>>> (daxctl-reconfigure-device)- this will allow changing the 'mode' of a
>>> dax device to kmem, online the resulting memory, and with your patches,
>>> also attempt to offline the memory, and change back to device-dax.
>>>
>>> In running with these patches, and testing the offlining part, I ran
>>> into the following lockdep below.
>>>
>>> This is with just these three patches on top of -rc7.
>>>
>>>
>>> [  +0.004886] ======================================================
>>> [  +0.001576] WARNING: possible circular locking dependency detected
>>> [  +0.001506] 5.1.0-rc7+ #13 Tainted: G           O
>>> [  +0.000929] ------------------------------------------------------
>>> [  +0.000708] daxctl/22950 is trying to acquire lock:
>>> [  +0.000548] 00000000f4d397f7 (kn->count#424){++++}, at: kernfs_remove_by_name_ns+0x40/0x80
>>> [  +0.000922]
>>>               but task is already holding lock:
>>> [  +0.000657] 000000002aa52a9f (mem_sysfs_mutex){+.+.}, at: unregister_memory_section+0x22/0xa0
>>
>> I have studied this issue, and now have a clear understanding why it
>> happens, I am not yet sure how to fix it, so suggestions are welcomed
>> :)
> 
> I would think that ACPI hotplug would have a similar problem, but it does this:
> 
>                 acpi_unbind_memory_blocks(info);
>                 __remove_memory(nid, info->start_addr, info->length);
> 
> I wonder if that ordering prevents going too deep into the
> device_unregister() call stack that you highlighted below.
> 

If that doesn't help, after we have

[PATCH v2 0/8] mm/memory_hotplug: Factor out memory block device handling

we could probably pull the memory device removal phase out from the
mem_hotplug_lock protection and let it be protected by the
device_hotplug_lock only. Might require some more work, though.

-- 

Thanks,

David / dhildenb