linux-kernel - Re: [PATCH 2/9] blkcg, writeback: Add wbc->no_wbc

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Message-ID: <20190624125856.GO657710@devbig004.ftw2.facebook.com>
Date:   Mon, 24 Jun 2019 05:58:56 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Jan Kara <jack@...e.cz>
Cc:     dsterba@...e.com, clm@...com, josef@...icpanda.com,
        axboe@...nel.dk, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 2/9] blkcg, writeback: Add wbc->no_wbc_acct

Hello, Jan.

On Mon, Jun 24, 2019 at 10:21:30AM +0200, Jan Kara wrote:
> OK, now I understand. Just one more question: So effectively, you are using
> wbc->no_wbc_acct to pass information from btrfs code to btrfs code telling
> it whether IO should or should not be accounted with wbc_account_io().

Yes.

> Wouldn't it make more sense to just pass this information internally
> within btrfs? Granted, if this mechanism gets more widespread use by other
> filesystems, then probably using wbc flag makes more sense. But I'm not
> sure if this isn't a premature generalization...

The btrfs async issuers end up using generic writeback path and uses
the generic wbc owner mechanisms so that ios are attached to the right
cgroup too.  So, I kinda prefer to provide a generic mechanism from
wbc side.  That said, the names are a bit misleading and I think it'd
be better to rename them to something more explicit, e.g. sth along
the line of wbc_update_cgroup_owner() and wbc->no_cgroup_owner.  What
do you think?

Thanks.

-- 
tejun