linux-kernel - Re: [v2 2/2] device-dax: "Hotremove" persistent memory that is used like normal RAM

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Date:   Thu, 25 Apr 2019 14:38:21 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        James Morris <jmorris@...ei.org>,
        Sasha Levin <sashal@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Keith Busch <keith.busch@...el.com>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Ross Zwisler <zwisler@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        "Huang, Ying" <ying.huang@...el.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Borislav Petkov <bp@...e.de>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Yaowei Bai <baiyaowei@...s.chinamobile.com>,
        Takashi Iwai <tiwai@...e.de>,
        Jérôme Glisse <jglisse@...hat.com>
Subject: Re: [v2 2/2] device-dax: "Hotremove" persistent memory that is used
 like normal RAM

On 25.04.19 14:30, Pavel Tatashin wrote:
>>
>> Yes, also I think you can let go of the device_lock in
>> check_memblocks_offline_cb, lock_device_hotplug() should take care of
>> this (see Documentation/core-api/memory-hotplug.rst - "locking internals")
>>
> Hi David,
> 
> Thank you for your comments. I went through memory-hotplug.rst, and I
> still think that device_lock() is needed here. In this particular case
> it can be replaced with something like READ_ONCE(), but for simplicity
> it is better to have device_lock()/device_unlock() as this is not a
> performance critical code.
> 
> I do not see any lock ordering issues with this code, as we are
> holding lock_device_hotplug() first that prevents userland from
> adding/removing memory during this check.

Yes, lock ordering is not an issue, I rather think that the device
hotplug lock will guard us in all situations. E.g. remove_memory() also
does not use it when checking if all blocks are offline. But you can
leave it in if you think it is needed.

> 
> https://soleen.com/source/xref/linux/arch/powerpc/platforms/powernv/memtrace.c?r=98fa15f3#248
> 
> Here we have a similar code:
> lock_device_hotplug();
>    online_mem_block();
>     device_online()
>      device_lock(dev);
> 
> Pasha
> 

-- 

Thanks,

David / dhildenb