Re: [PATCH V6 7/8] fs/xfs: Change xfs_ioctl_setattr_dax_invalidate() to xfs_ioctl_dax_check()

From: Jan Kara <jack@suse.cz>
To: Dave Chinner <david@fromorbit.com>
Cc: ira.weiny@intel.com, linux-kernel@vger.kernel.org,
	"Darrick J. Wong" <darrick.wong@oracle.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Christoph Hellwig <hch@lst.de>,
	"Theodore Y. Ts'o" <tytso@mit.edu>, Jan Kara <jack@suse.cz>,
	Jeff Moyer <jmoyer@redhat.com>,
	linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH V6 7/8] fs/xfs: Change xfs_ioctl_setattr_dax_invalidate() to xfs_ioctl_dax_check()
Date: Wed, 8 Apr 2020 11:58:03 +0200	[thread overview]
Message-ID: <20200408095803.GB30172@quack2.suse.cz> (raw)
In-Reply-To: <20200408022318.GJ24067@dread.disaster.area>

On Wed 08-04-20 12:23:18, Dave Chinner wrote:
> On Tue, Apr 07, 2020 at 11:29:57AM -0700, ira.weiny@intel.com wrote:
> > From: Ira Weiny <ira.weiny@intel.com>
> > 
> > We only support changing FS_XFLAG_DAX on directories.  Files get their
> > flag from the parent directory on creation only.  So no data
> > invalidation needs to happen.
> 
> Which leads me to ask: how are users and/or admins supposed to
> remove the flag from regular files once it is set in the filesystem?
> 
> Only being able to override the flag via the "dax=never" mount
> option means that once the flag is set, nobody can ever remove it
> and they can only globally turn off dax if it gets set incorrectly.
> It also means a global interrupt because all apps on the filesystem
> need to be stopped so the filesystem can be unmounted and mounted
> again with dax=never. This is highly unfriendly to admins and users.
> 
> IOWs, we _must_ be able to clear this inode flag on regular inodes
> in some way. I don't care if it doesn't change the current in-memory
> state, but we must be able to clear the flags so that the next time
> the inodes are instantiated DAX is not enabled for those files...

Well, there's one way to clear the flag: delete the file. If you still care
about the data, you can copy the data first. It isn't very convenient, I
agree, and effectively means restarting whatever application that is using
the file. But it seems like more understandable API than letting user clear
the on-disk flag but the inode will still use DAX until kernel decides to
evict the inode - because that often means you need to restart the
application using the file anyway for the flag change to have any effect.

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