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From: Ira Weiny <ira.weiny@intel.com>
To: Dave Chinner <david@fromorbit.com>
Cc: Jan Kara <jack@suse.cz>,
	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>,
	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 15:26:36 -0700	[thread overview]
Message-ID: <20200408222636.GC664132@iweiny-DESK2.sc.intel.com> (raw)
In-Reply-To: <20200408210950.GL24067@dread.disaster.area>

On Thu, Apr 09, 2020 at 07:09:50AM +1000, Dave Chinner wrote:
> On Wed, Apr 08, 2020 at 11:58:03AM +0200, Jan Kara wrote:
> > 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.
> 
> Restarting the application is fine. Having to backup/restore or copy
> the entire data set just to turn off an inode flag? That's not a
> viable management strategy. We could be talking about terabytes of
> data here.
> 
> I explained how we can safely remove the flag in the other branch of
> this thread...
> 
> > 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
> 
> Certainly doesn't seem that way to me. "stop app, clear flags, drop
> caches, restart app" is a pretty simple, easy thing to do for an
> admin.

I want to be clear here: I think this is reasonable.  However, I don't see
consensus for that interface.

Christoph in particular said that a 'lazy change' is: "... straight from
the playbook for arcane and confusing API designs."

	"But returning an error and doing a lazy change anyway is straight from
	the playbook for arcane and confusing API designs."

	-- https://lore.kernel.org/lkml/20200403072731.GA24176@lst.de/

Did I somehow misunderstand this?

Again for this patch set, 5.8, lets leave that alone for now.  I think if we
disable setting this on files right now we can still allow it in the future as
another step forward.

> 
> Especially compared to process that is effectively "stop app, backup
> data set, delete data set, clear flags, restore data set, restart
> app"
> 
> > - because that often means you need to restart the
> > application using the file anyway for the flag change to have any effect.
> 
> That's a trivial requirement compared to the downtime and resource
> cost of a data set backup/restore just to clear inode flags....
> 

I agree but others do not.  This still provides a baby step forward and some
granularity for those who plan out the creation of their files.

Ira


  reply	other threads:[~2020-04-08 22:26 UTC|newest]

Thread overview: 48+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-07 18:29 [PATCH V6 0/8] Enable per-file/per-directory DAX operations V6 ira.weiny
2020-04-07 18:29 ` [PATCH V6 1/8] fs/xfs: Remove unnecessary initialization of i_rwsem ira.weiny
2020-04-07 23:46   ` Dave Chinner
2020-04-07 18:29 ` [PATCH V6 2/8] fs: Remove unneeded IS_DAX() check ira.weiny
2020-04-09  7:31   ` Christoph Hellwig
2020-04-09 14:57     ` Ira Weiny
2020-04-07 18:29 ` [PATCH V6 3/8] fs/stat: Define DAX statx attribute ira.weiny
2020-04-07 23:47   ` Dave Chinner
2020-04-07 18:29 ` [PATCH V6 4/8] fs/xfs: Make DAX mount option a tri-state ira.weiny
2020-04-07 23:59   ` Dave Chinner
2020-04-08  0:09     ` Ira Weiny
2020-04-08  0:48       ` Dave Chinner
2020-04-09 15:03         ` Ira Weiny
2020-04-07 18:29 ` [PATCH V6 5/8] fs/xfs: Create function xfs_inode_enable_dax() ira.weiny
2020-04-08  0:05   ` Dave Chinner
2020-04-08  0:13     ` Ira Weiny
2020-04-07 18:29 ` [PATCH V6 6/8] fs/xfs: Combine xfs_diflags_to_linux() and xfs_diflags_to_iflags() ira.weiny
2020-04-08  2:08   ` Dave Chinner
2020-04-08 17:09     ` Ira Weiny
2020-04-08 21:02       ` Dave Chinner
2020-04-08 21:28         ` Dan Williams
2020-04-08 22:10           ` Ira Weiny
2020-04-08 23:58           ` Dave Chinner
2020-04-09  0:22             ` Ira Weiny
2020-04-09 12:41               ` Christoph Hellwig
2020-04-09 20:49                 ` Ira Weiny
2020-04-08 22:07         ` Ira Weiny
2020-04-08 23:21           ` Dave Chinner
2020-04-09  0:12             ` Ira Weiny
2020-04-09  0:30               ` Darrick J. Wong
2020-04-09 15:29                 ` Ira Weiny
2020-04-09 16:59                   ` Darrick J. Wong
2020-04-09 17:17                     ` Jan Kara
2020-04-09 20:54                     ` Ira Weiny
2020-04-09  0:49               ` Dave Chinner
2020-04-09 12:40                 ` Christoph Hellwig
2020-04-10  0:27                   ` Dave Chinner
2020-04-07 18:29 ` [PATCH V6 7/8] fs/xfs: Change xfs_ioctl_setattr_dax_invalidate() to xfs_ioctl_dax_check() ira.weiny
2020-04-08  2:23   ` Dave Chinner
2020-04-08  9:58     ` Jan Kara
2020-04-08 21:09       ` Dave Chinner
2020-04-08 22:26         ` Ira Weiny [this message]
2020-04-08 23:48           ` Dave Chinner
2020-04-09 12:28             ` Christoph Hellwig
2020-04-08 15:37   ` Darrick J. Wong
2020-04-08 18:13     ` Ira Weiny
2020-04-16  5:39   ` [fs/xfs] 857c9841f8: xfstests.xfs.046.fail kernel test robot
2020-04-07 18:29 ` [PATCH V6 8/8] Documentation/dax: Update Usage section ira.weiny

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