linux-kernel - Re: [PATCH V6 7/8] fs/xfs: Change xfs_ioctl_setattr_dax_invalidate() to xfs_ioctl_dax

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Date:   Wed, 8 Apr 2020 12:23:18 +1000
From:   Dave Chinner <david@...morbit.com>
To:     ira.weiny@...el.com
Cc:     linux-kernel@...r.kernel.org,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        Jeff Moyer <jmoyer@...hat.com>, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH V6 7/8] fs/xfs: Change xfs_ioctl_setattr_dax_invalidate()
 to xfs_ioctl_dax_check()

On Tue, Apr 07, 2020 at 11:29:57AM -0700, ira.weiny@...el.com wrote:
> From: Ira Weiny <ira.weiny@...el.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...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com