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

From: David Hildenbrand <david@redhat.com>
To: Pavel Tatashin <pasha.tatashin@soleen.com>,
	Dan Williams <dan.j.williams@intel.com>
Cc: "Sasha Levin" <sashal@kernel.org>,
	"Tom Lendacky" <thomas.lendacky@amd.com>,
	"Michal Hocko" <mhocko@suse.com>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	"Takashi Iwai" <tiwai@suse.de>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"Huang, Ying" <ying.huang@intel.com>,
	"James Morris" <jmorris@namei.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"Linux MM" <linux-mm@kvack.org>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Borislav Petkov" <bp@suse.de>,
	"Yaowei Bai" <baiyaowei@cmss.chinamobile.com>,
	"Ross Zwisler" <zwisler@kernel.org>,
	"Bjorn Helgaas" <bhelgaas@google.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Fengguang Wu" <fengguang.wu@intel.com>
Subject: Re: [v2 2/2] device-dax: "Hotremove" persistent memory that is used like normal RAM
Date: Thu, 25 Apr 2019 09:41:20 +0200	[thread overview]
Message-ID: <180d6250-8a6a-0b5d-642a-ec6648cb45b1@redhat.com> (raw)
In-Reply-To: <CA+CK2bDB5o4+NMc7==_ipVAZoEo7fdrkjZ4etU0LUCqxnmN-Rg@mail.gmail.com>

On 24.04.19 23:34, Pavel Tatashin wrote:
>>>> +static int
>>>> +offline_memblock_cb(struct memory_block *mem, void *arg)
>>>
>>> Function name suggests that you are actually trying to offline memory
>>> here. Maybe check_memblocks_offline_cb(), just like we have in
>>> mm/memory_hotplug.c.
> 
> Makes sense, I will rename to check_memblocks_offline_cb()
> 
>>>> +     lock_device_hotplug();
>>>> +     rc = walk_memory_range(start_pfn, end_pfn, dev, offline_memblock_cb);
>>>> +     unlock_device_hotplug();
>>>> +
>>>> +     /*
>>>> +      * If admin has not offlined memory beforehand, we cannot hotremove dax.
>>>> +      * Unfortunately, because unbind will still succeed there is no way for
>>>> +      * user to hotremove dax after this.
>>>> +      */
>>>> +     if (rc)
>>>> +             return rc;
>>>
>>> Can't it happen that there is a race between you checking if memory is
>>> offline and an admin onlining memory again? maybe pull the
>>> remove_memory() into the locked region, using __remove_memory() instead.
>>
>> I think the race is ok. The admin gets to keep the pieces of allowing
>> racing updates to the state and the kernel will keep the range active
>> until the next reboot.
> 
> Thank you for noticing this. I will pull it into locking region.
> Because, __remove_memory() has this code:
> 
> 1868   ret = walk_memory_range(PFN_DOWN(start), PFN_UP(start + size - 1), NULL,
> 1869   check_memblock_offlined_cb);
> 1870   if (ret)
> 1871      BUG();
> 

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")

Cheers!

-- 

Thanks,

David / dhildenb
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