From: Ira Weiny <email@example.com>
To: "Darrick J. Wong" <firstname.lastname@example.org>
Cc: Dave Chinner <email@example.com>,
Dan Williams <firstname.lastname@example.org>,
Christoph Hellwig <email@example.com>,
"Theodore Y. Ts'o" <firstname.lastname@example.org>, Jan Kara <email@example.com>,
Jeff Moyer <firstname.lastname@example.org>,
Subject: Re: [PATCH V6 6/8] fs/xfs: Combine xfs_diflags_to_linux() and xfs_diflags_to_iflags()
Date: Thu, 9 Apr 2020 08:29:44 -0700 [thread overview]
Message-ID: <20200409152944.GA801705@iweiny-DESK2.sc.intel.com> (raw)
On Wed, Apr 08, 2020 at 05:30:21PM -0700, Darrick J. Wong wrote:
> But you're right, this thing keeps swirling around and around and around
> because we can't ever get to agreement on this. Maybe I'll just become
> XFS BOFH MAINTAINER and make a decision like this:
> 1 Applications must call statx to discover the current S_DAX state.
> 2 There exists an advisory file inode flag FS_XFLAG_DAX that is set based on
> the parent directory FS_XFLAG_DAX inode flag. This advisory flag can be
> changed after file creation, but it does not immediately affect the S_DAX
> If FS_XFLAG_DAX is set and the fs is on pmem then it will enable S_DAX at
> inode load time; if FS_XFLAG_DAX is not set, it will not enable S_DAX.
> Unless overridden...
> 3 There exists a dax= mount option.
> "-o dax=never" means "never set S_DAX, ignore FS_XFLAG_DAX"
> "-o dax=always" means "always set S_DAX (at least on pmem), ignore FS_XFLAG_DAX"
> "-o dax" by itself means "dax=always"
> "-o dax=iflag" means "follow FS_XFLAG_DAX" and is the default
per-Dave '-o dax=inode'
> 4 There exists an advisory directory inode flag FS_XFLAG_DAX that can be
> changed at any time. The flag state is copied into any files or
> subdirectories when they are created within that directory.
> If programs
> require file access runs in S_DAX mode, they must create those files
> inside a directory with FS_XFLAG_DAX set, or mount the fs with an
> appropriate dax mount option.
Why do we need this to be true? If the FS_XFLAG_DAX flag can be cleared why
not set it and allow the S_DAX change to occur later just like clearing it?
The logic is exactly the same.
> 5 Programs that require a specific file access mode (DAX or not DAX) must
> do one of the following:
> (a) create files in directories with the FS_XFLAG_DAX flag set as needed;
Again if we allow clearing the flag why not setting? So this is 1 option they
> (b) have the administrator set an override via mount option;
> (c) if they need to change a file's FS_XFLAG_DAX flag so that it does not
> match the S_DAX state (as reported by statx), they must cause the
> kernel to evict the inode from memory. This can be done by:
> i> closing the file;
> ii> re-opening the file and using statx to see if the fs has
> changed the S_DAX flag;
i and ii need to be 1 step the user must follow.
> iii> if not, either unmount and remount the filesystem, or
> closing the file and using drop_caches.
> 6 I no longer think it's too wild to require that users who want to
> squeeze every last bit of performance out of the particular rough and
> tumble bits of their storage also be exposed to the difficulties of
> what happens when the operating system can't totally virtualize those
> hardware capabilities. Your high performance sports car is not a
> Toyota minivan, as it were.
I'm good with this statement. But I think we need to clean up the verbiage for
the documentation... ;-)
Thanks for the summary. I like these to get everyone on the same page. :-D
> I think (like Dave said) that if you set XFS_IDONTCACHE on the inode
> when you change the DAX flag, the VFS will kill the inode the instant
> the last user close()s the file. Then 5.c.ii will actually work.
> > >
> > > > Furthermore, if we did want an interface like that why not allow
> > > > the on-disk flag to be set as well as cleared?
> > >
> > > Well, why not - it's why I implemented the flag in the first place!
> > > The only problem we have here is how to safely change the in-memory
> > > DAX state, and that largely has nothing to do with setting/clearing
> > > the on-disk flag....
> > With the above change to xfs_diflags_to_iflags() I think we are ok here.
> > Ira
next prev parent reply other threads:[~2020-04-09 15:29 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 [this message]
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
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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