> Am 09.05.2020 um 01:53 schrieb Andrew Morton <[email protected]>:
>
> On Fri, 8 May 2020 10:42:14 +0200 David Hildenbrand <[email protected]>
> wrote:
>
>> Assume we have kmem configured and loaded:
>> [root@localhost ~]# cat /proc/iomem
>> ...
>> 140000000-33fffffff : Persistent Memory$
>> 140000000-1481fffff : namespace0.0
>> 150000000-33fffffff : dax0.0
>> 150000000-33fffffff : System RAM
>>
>> Assume we try to unload kmem. This force-unloading will work, even if
>> memory cannot get removed from the system.
>> [root@localhost ~]# rmmod kmem
>> [ 86.380228] removing memory fails, because memory
>> [0x0000000150000000-0x0000000157ffffff] is onlined
>> ...
>> [ 86.431225] kmem dax0.0: DAX region [mem 0x150000000-0x33fffffff] cannot
>> be hotremoved until the next reboot
>>
>> Now, we can reconfigure the namespace:
>> [root@localhost ~]# ndctl create-namespace --force --reconfig=namespace0.0
>> --mode=devdax
>> [ 131.409351] nd_pmem namespace0.0: could not reserve region [mem
>> 0x140000000-0x33fffffff]dax
>> [ 131.410147] nd_pmem: probe of namespace0.0 failed with error
>> -16namespace0.0 --mode=devdax
>> ...
>>
>> This fails as expected due to the busy memory resource, and the memory
>> cannot be used. However, the dax0.0 device is removed, and along its name.
>>
>> The name of the memory resource now points at freed memory (name of the
>> device).
>> [root@localhost ~]# cat /proc/iomem
>> ...
>> 140000000-33fffffff : Persistent Memory
>> 140000000-1481fffff : namespace0.0
>> 150000000-33fffffff : �_�^7_��/_��wR��WQ���^��� ...
>> 150000000-33fffffff : System RAM
>>
>> We have to make sure to duplicate the string. While at it, remove the
>> superfluous setting of the name and fixup a stale comment.
>>
>> Fixes: 9f960da72b25 ("device-dax: "Hotremove" persistent memory that is used
>> like normal RAM")
>> Cc: [email protected] # v5.3
>
> hm.
>
> Is this really -stable material? These are all privileged operations,
> I expect?
Yes, my thought was rather that an admin could bring the system into such a
state (by mistake?). Let‘s see if somebody has a suggestion.
I guess if we were really unlucky, we could access invalid memory and trigger a
BUG (e.g., page at the end of memory and does not contain a 0 byte).
>
> Assuming "yes", I've queued this separately, staged for 5.7-rcX. I'll
> redo patches 2-4 as a three-patch series for 5.8-rc1.
Make sense, let‘s wait for review feedback, thanks!
