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

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Date:   Tue, 19 Dec 2017 14:04:54 +0100
From:   Jan Kara <jack@...e.cz>
To:     Tejun Heo <tj@...nel.org>
Cc:     Jan Kara <jack@...e.cz>, Jens Axboe <axboe@...nel.dk>,
        cgroups@...r.kernel.org, xuejiufei <xuejiufei@...il.com>,
        kernel-team@...com, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2] writeback: synchronize sync(2) against cgroup
 writeback membership switches

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@...e.cz>

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