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From: Ira Weiny <ira.weiny@intel.com>
To: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: Dave Chinner <david@fromorbit.com>,
	linux-kernel@vger.kernel.org,
	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 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)
In-Reply-To: <20200409003021.GJ6742@magnolia>

On Wed, Apr 08, 2020 at 05:30:21PM -0700, Darrick J. Wong wrote:

[snip]

> 
> 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
>    state.
> 
>    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.

Good.

>    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

s/must/can/

>    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
'can' do.

> 
>    (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
Ira

> 
> 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.
> 
> --D
> 
> > > 
> > > > 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
> > 

  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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