linux-kernel - Re: [PATCH] writeback, cgroup: switch inodes with dirty timestamps to release dying cgwbs

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Message-ID: <ZSgtW0wGZZ3N3oKl@slm.duckdns.org>
Date:   Thu, 12 Oct 2023 07:31:07 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Jingbo Xu <jefflexu@...ux.alibaba.com>
Cc:     guro@...com, lizefan.x@...edance.com, hannes@...xchg.org,
        cgroups@...r.kernel.org, jack@...e.cz,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        viro@...iv.linux.org.uk, brauner@...nel.org, willy@...radead.org,
        joseph.qi@...ux.alibaba.com
Subject: Re: [PATCH] writeback, cgroup: switch inodes with dirty timestamps
 to release dying cgwbs

On Wed, Oct 11, 2023 at 04:42:28PM +0800, Jingbo Xu wrote:
> The cgwb cleanup routine will try to release the dying cgwb by switching
> the attached inodes.  It fetches the attached inodes from wb->b_attached
> list, omitting the fact that inodes only with dirty timestamps reside in
> wb->b_dirty_time list, which is the case when lazytime is enabled.  This
> causes enormous zombie memory cgroup when lazytime is enabled, as inodes
> with dirty timestamps can not be switched to a live cgwb for a long time.
> 
> It is reasonable not to switch cgwb for inodes with dirty data, as
> otherwise it may break the bandwidth restrictions.  However since the
> writeback of inode metadata is not accounted, let's also switch inodes
> with dirty timestamps to avoid zombie memory and block cgroups when
> laztytime is enabled.
> 
> Fixs: c22d70a162d3 ("writeback, cgroup: release dying cgwbs by switching attached inodes")
> Signed-off-by: Jingbo Xu <jefflexu@...ux.alibaba.com>

The patch looks fine to me.

...
> +	restart = isw_prepare_wbs_switch(isw, &wb->b_attached, &nr);
> +	if (!restart)
> +		restart = isw_prepare_wbs_switch(isw, &wb->b_dirty_time, &nr);

But can you add a comment explaining why we're also migrating b_dirty_time?

Thanks.

-- 
tejun