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From: Ira Weiny <ira.weiny@intel.com>
To: Dave Chinner <david@fromorbit.com>
Cc: linux-kernel@vger.kernel.org,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	"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>,
	linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 5/5] fs/xfs: Allow toggle of physical DAX flag
Date: Mon, 21 Oct 2019 15:49:31 -0700	[thread overview]
Message-ID: <20191021224931.GA25526@iweiny-DESK2.sc.intel.com> (raw)
In-Reply-To: <20191021004536.GD8015@dread.disaster.area>

On Mon, Oct 21, 2019 at 11:45:36AM +1100, Dave Chinner wrote:
> On Sun, Oct 20, 2019 at 08:59:35AM -0700, ira.weiny@intel.com wrote:
> > @@ -1232,12 +1233,10 @@ xfs_diflags_to_linux(
> >  		inode->i_flags |= S_NOATIME;
> >  	else
> >  		inode->i_flags &= ~S_NOATIME;
> > -#if 0	/* disabled until the flag switching races are sorted out */
> >  	if (xflags & FS_XFLAG_DAX)
> >  		inode->i_flags |= S_DAX;
> >  	else
> >  		inode->i_flags &= ~S_DAX;
> > -#endif
> 
> This code has bit-rotted. See xfs_setup_iops(), where we now have a
> different inode->i_mapping->a_ops for DAX inodes.

:-(

> 
> That, fundamentally, is the issue here - it's not setting/clearing
> the DAX flag that is the issue, it's doing a swap of the
> mapping->a_ops while there may be other code using that ops
> structure.
> 
> IOWs, if there is any code anywhere in the kernel that
> calls an address space op without holding one of the three locks we
> hold here (i_rwsem, MMAPLOCK, ILOCK) then it can race with the swap
> of the address space operations.
> 
> By limiting the address space swap to file sizes of zero, we rule
> out the page fault path (mmap of a zero length file segv's with an
> access beyond EOF on the first read/write page fault, right?).

Yes I checked that and thought we were safe here...

> However, other aops callers that might run unlocked and do the wrong
> thing if the aops pointer is swapped between check of the aop method
> existing and actually calling it even if the file size is zero?
> 
> A quick look shows that FIBMAP (ioctl_fibmap())) looks susceptible
> to such a race condition with the current definitions of the XFS DAX
> aops. I'm guessing there will be others, but I haven't looked
> further than this...

I'll check for others and think on what to do about this.  ext4 will have the
same problem I think.  :-(

I don't suppose using a single a_ops for both DAX and non-DAX is palatable?

> 
> >  	/* lock, flush and invalidate mapping in preparation for flag change */
> >  	xfs_ilock(ip, XFS_MMAPLOCK_EXCL | XFS_IOLOCK_EXCL);
> > +
> > +	if (i_size_read(inode) != 0) {
> > +		error = -EOPNOTSUPP;
> > +		goto out_unlock;
> > +	}
> 
> Wrong error. Should be the same as whatever is returned when we try
> to change the extent size hint and can't because the file is
> non-zero in length (-EINVAL, I think). Also needs a comment
> explainging why this check exists, and probably better written as
> i_size_read() > 0 ....

Done, done, and done.

Thank you,
Ira

> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com

  reply	other threads:[~2019-10-21 22:49 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-20 15:59 [PATCH 0/5] Enable per-file/directory DAX operations ira.weiny
2019-10-20 15:59 ` [PATCH 1/5] fs/stat: Define DAX statx attribute ira.weiny
2019-10-22 11:32   ` Boaz Harrosh
2019-10-22 16:51     ` Ira Weiny
2019-10-20 15:59 ` [PATCH 2/5] fs/xfs: Isolate the physical DAX flag from effective ira.weiny
2019-10-21  0:26   ` Dave Chinner
2019-10-21 17:40     ` Ira Weiny
2019-10-20 15:59 ` [PATCH 3/5] fs/xfs: Separate functionality of xfs_inode_supports_dax() ira.weiny
2019-10-20 15:59 ` [PATCH 4/5] fs/xfs: Clean up DAX support check ira.weiny
2019-10-20 15:59 ` [PATCH 5/5] fs/xfs: Allow toggle of physical DAX flag ira.weiny
2019-10-21  0:45   ` Dave Chinner
2019-10-21 22:49     ` Ira Weiny [this message]
2019-10-21 23:46       ` Dave Chinner
2019-11-08 13:12       ` Jan Kara
2019-11-08 13:46         ` Jan Kara
2019-11-08 19:36           ` Ira Weiny
2019-11-11 16:07             ` Jan Kara
2019-11-11 23:54               ` Ira Weiny
2019-10-22 11:21 ` [PATCH 0/5] Enable per-file/directory DAX operations Boaz Harrosh
2019-10-23 13:09   ` Boaz Harrosh
2019-10-23 22:13     ` Dave Chinner
2019-10-24  2:31       ` Boaz Harrosh
2019-10-24  7:34         ` Dave Chinner
2019-10-24 14:05           ` Boaz Harrosh
2019-10-24 21:35             ` Dave Chinner
2019-10-24 23:29               ` Boaz Harrosh
2019-10-25  0:36                 ` Dave Chinner
2019-10-25  1:15                   ` Boaz Harrosh
2019-10-25 20:49                   ` Ira Weiny
2019-10-27 22:10                     ` Dave Chinner
2019-10-31 16:17                       ` Ira Weiny
2019-11-01 22:47                         ` Dave Chinner
2019-11-02  4:25                           ` Dan Williams

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