[dm-devel] dm thin-volume hung as swap: bug or as-design ?

[Coly Li]

On 1/29/21 9:57 PM, Alasdair G Kergon wrote:
> On Fri, Jan 29, 2021 at 06:40:06PM +0800, Coly Li wrote:
>> Recently I receive a report that whole system hung and no response after
>> a while with I/O load. The special configuration is the dm thin-pool
>> volume is used as the swap partition of the system.
>> My questions are,
>> - Can a thin-pool volume be used as swap device?
> 
> Yes in principle, but it won't get much testing as it's not 
> necessarily a particularly sensible configuration.
> - You'd normally prefer fully-pre-allocated disk space for swap
>   and turn off the zeroing.
> 
> Is there some use-case where it does make more sense?
> 

What I see is on a system there are dozens of partitions created on top
of many thin-pool for each, including the swap partition. People just
use thin-pool volumes in this way, and bug triggered.


>> - The above description is a bug, or an already know issue which should
>> be avoided ?
>  
> Bug.
> 

Thanks for the confirmation. I tried to change all memory allocation
into GFP_NOIO or with memalloc_noio_save(), the deadlock still exists.
What I suspect yet is might be from the memory allocation from the
kworkers, but this is only my guess and no evidence.

If there is patch addressed this hung issue (thin-pool volume as swap),
I'd like to help testing, because my local environment may 100%
reproduce the problem in 5 minutes.

Thanks.

Coly Li