Re: Request For Suggestion: how to handle udevd timeout for bcache registration

[Jens Axboe <axboe@kernel.dk>]

On 4/23/20 5:23 AM, Coly Li wrote:
> Hi folk,
> 
> I want to listen to your suggestion on how to handle the udevd timeout
> for bcache registration.
> 
> First of all let me introduce the background of this timeout problem.
> 
> Now the bcache registration is synchronized, the registering process
> will be blocked until the whole registration done. In boot up time, such
> registration can be initiated from a bcache udev rule. Normally it won't
> be problem, but for very large cached data size there might be a large
> internal btree on the cache device. During the registration checking all
> the btree nodes may take 50+ minutes as a udev task, it exceeds 180
> seconds timeout and udevd will kill it. The killing signal will make
> kthread_create() fail during bcache initialization, then the automatic
> bcache registration in boot up time will fail.
> 
> The above text describes the problem I need to solve: make boot up time
> automatic bache registration always success no mater how long it will take.
> 
> I know there are several solutions to solve such problem, I do
> appreciate if you may share the solution so that I may learn good ideas
> from them.
> 
> Thank you in advance for the information sharing of my request of
> suggestion.

The way I see it, you have only two choices:

1) Make the registration async (or lazy), so that starting the device is
   fast, but the btree verification happens on-demand or in the
   background.

2) Increase udev timeout.

That's about it, I don't think there's any clever tricks to be had, and
I definitely don't want to go down the path of trying to work around the
udev killing in the kernel.

-- 
Jens Axboe