Re: Question about forcing 'disable-memdev'

[Jane Chu <jane.chu@oracle.com>]

On 2/27/2024 12:28 PM, Dave Jiang wrote:

>
> On 2/27/24 1:24 PM, Jane Chu wrote:
>> On 2/27/2024 8:40 AM, Dave Jiang wrote:
>>
>>> On 2/26/24 10:32 PM, Cao, Quanquan/曹 全全 wrote:
>>>> Hi, Dave
>>>>
>>>> On the basis of this patch, I conducted some tests and encountered unexpected errors. I would like to inquire whether the design here is reasonable? Below are the steps of my testing:
>>>>
>>>> Link: https://lore.kernel.org/linux-cxl/170138109724.2882696.123294980050048623.stgit@djiang5-mobl3/
>>>>
>>>>
>>>> Problem description: after creating a region, directly forcing 'disable-memdev' and then consuming memory leads to a kernel panic.
>>> If you are forcing memory disable when the memory cannot be offlined, then this behavior is expected. You are ripping the memory away from underneath kernel mm. The reason the check was added is to prevent the users from doing exactly that.
>> Since user is doing the illegal thing, shouldn't that lead to SIGBUS or SIGKILL ?
> The behavior is unpredictable once the CXL memory is ripped away. If the memory only backed user memory then you may see SIGBUS. But if the memory backed kernel data then kernel OOPs is not out of question.

Make sense, thanks for the clarification.

-jane

>
>> thanks,
>>
>> -jane
>>
>>>> Test environment:
>>>> KERNEL    6.8.0-rc1
>>>> QEMU    8.2.0-rc4
>>>>
>>>> Test steps：
>>>>         step1: set memory auto_online to movable zones.
>>>>              echo online_movable > /sys/devices/system/memory/auto_online_blocks
>>>>         step2: create region
>>>>              cxl create-region -t ram -d decoder0.0 -m mem0
>>>>         step3: disable memdev
>>>>              cxl disable-memdev mem0 -f
>>>>         step4: consum CXL memory
>>>>              ./consumemem   <------kernel panic
>>>>
>>>> numactl node status:
>>>>         step1: numactl -H
>>>>
>>>>       available: 2 nodes (0-1)
>>>>       node 0 cpus: 0 1
>>>>       node 0 size: 968 MB
>>>>       node 0 free: 664 MB
>>>>       node 1 cpus: 2 3
>>>>       node 1 size: 683 MB
>>>>       node 1 free: 333 MB
>>>>       node distances:
>>>>       node   0   1
>>>>         0:  10  20
>>>>
>>>>       step2: numactl -H
>>>>
>>>>       available: 3 nodes (0-2)
>>>>       node 0 cpus: 0 1
>>>>       node 0 size: 968 MB
>>>>       node 0 free: 677 MB
>>>>       node 1 cpus: 2 3
>>>>       node 1 size: 683 MB
>>>>       node 1 free: 333 MB
>>>>       node 2 cpus:
>>>>       node 2 size: 256 MB
>>>>       node 2 free: 256 MB
>>>>       node distances:
>>>>       node   0   1   2
>>>>         0:  10  20  20
>>>>         1:  20  10  20
>>>>         2:  20  20  10
>>>>
>>>>       step3: numactl -H
>>>>
>>>>       available: 3 nodes (0-2)
>>>>       node 0 cpus: 0 1
>>>>       node 0 size: 968 MB
>>>>       node 0 free: 686 MB
>>>>       node 1 cpus: 2 3
>>>>       node 1 size: 683 MB
>>>>       node 1 free: 336 MB
>>>>       node 2 cpus:
>>>>       node 2 size: 256 MB
>>>>       node 2 free: 256 MB
>>>>       node distances:
>>>>       node   0   1   2
>>>>         0:  10  20  20
>>>>         1:  20  10  20
>>>>         2:  20  20  10