Re: [PATCH] btrfs: limit async_work allocation and worker func duration

["Holger Hoffstätte" <holger@applied-asynchrony.com>]

On Mon, 12 Dec 2016 15:54:43 +0100, David Sterba wrote:

> On Fri, Dec 02, 2016 at 05:51:36PM -0800, Maxim Patlasov wrote:
>> Problem statement: unprivileged user who has read-write access to more than
>> one btrfs subvolume may easily consume all kernel memory (eventually
>> triggering oom-killer).
>> 
[..snip..]
>>  
>> +bool btrfs_workqueue_normal_congested(struct btrfs_workqueue *wq)
>> +{
>> +	int thresh = wq->normal->thresh != NO_THRESHOLD ?
>> +		wq->normal->thresh : num_possible_cpus();
> 
> Why not num_online_cpus? I vaguely remember we should be checking online
> cpus, but don't have the mails for reference. We use it elsewhere for

+1

> spreading the work over cpus, but it's still not bullet proof regarding
> cpu onlining/offlining.
> 
> Otherwise looks good to me, as far as I can imagine the possible
> behaviour of the various async parameters just from reading the code.

If it's any help I have been running with this for a few days now; regular
day-to-day work, snapshots, balancing, defrags etc. with no obvious
problems, though I haven't tried to break it with the reproducer either.
Anyway:

Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>

cheers,
Holger