linux-ext4 - Re: [PATCH 2/3] vfs, writeback: replace FS_CGROUP_WRITEBACK with MS

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Date:	Mon, 15 Jun 2015 13:39:49 +0200
From:	Jan Kara <jack@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	Christoph Hellwig <hch@...radead.org>, axboe@...nel.dk,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	lizefan@...wei.com, cgroups@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 2/3] vfs, writeback: replace FS_CGROUP_WRITEBACK with
 MS_CGROUPWB

On Sun 14-06-15 00:42:36, Tejun Heo wrote:
> Hello, Christoph.
> 
> On Sat, Jun 13, 2015 at 09:16:08AM -0700, Christoph Hellwig wrote:
> > On Fri, Jun 12, 2015 at 04:57:33PM -0500, Tejun Heo wrote:
> > > FS_CGROUP_WRITEBACK indicates whether a file_system_type supports
> > > cgroup writeback; however, different super_blocks of the same
> > > file_system_type may or may not support cgroup writeback depending on
> > > filesystem options.  This patch replaces FS_CGROUP_WRITEBACK with a
> > > kernel-internal super_block->s_flags MS_CGROUPWB.  The concatenated
> > > and abbreviated name is for consistency with other MS_* flags.
> > 
> > Nak.  As the uapi part makes it obvious the MS_ namespace is part
> > of the userspace ABI.  Please add a new in-kernel flags field instead.
> 
> Are MS_ACTIVE and MS_BORN part of userpace ABI?  They seem pretty
> internal.  I don't mind introducing a new internal flag field but it's
> weird to put this single flag there with other internal flags in
> ->s_flags.
  So you are right that there are other internal flags allocated from the
top of the i_flags field, however we are pretty much running out of the
flags available for the ABI so it's better to move internal flags elsewhere
as that's simpler than creating a new ABI for mount...

> Assuming we add a new field, how do sb->s_iflags and SB_I_XXX sound?
> Any better suggestions?
  Looks good to me.

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