Re: [PATCH v2] writeback: synchronize sync(2) against cgroup writeback membership switches

Jan Kara
2017-12-19T13:05:18Z

On Wed 13-12-17 07:39:30, Tejun Heo wrote:
> Hello,
>
> On Wed, Dec 13, 2017 at 12:00:04PM +0100, Jan Kara wrote:
> > OK, but this effectively prevents writeback from sync_inodes_sb() to ever
> > make inode switch wbs. Cannot that be abused in some way like making sure
> > writeback of our memcg is "invisible" by forcing it out using sync(2)? It
> > just looks a bit dangerous to me...
>
> That's true. There are a couple mitigating factors tho.
>
> * While it can delay switching during sync(2), it'll all still be
> recorded and the switch will happen soon if needed.
>
> * sync(2) is hugely disruptive with or without this and can easily be
> used to DOS the whole system. People are working on restricting the
> blast radius of sync(2) to mitigate this problem, which most likely
> make this a non-problem too.
>
> If you can think of a better solution, I'm all ears.

After some thinking about this I don't have a better solution. So you can
add:

Acked-by: Jan Kara <jack@suse.cz>

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR