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

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Message-ID: <20150614054236.GA9662@mtj.duckdns.org>
Date:	Sun, 14 Jun 2015 00:42:36 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	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

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.

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

Thanks.

-- 
tejun
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