Re: [PATCH 4/7] xfs: protect S_DAX transitions in XFS write path

[Dave Chinner]

On Mon, Sep 25, 2017 at 05:14:01PM -0600, Ross Zwisler wrote:
> In the current XFS write I/O path we check IS_DAX() in
> xfs_file_write_iter() to decide whether to do DAX I/O, direct I/O or
> buffered I/O.  This check is done without holding the XFS_IOLOCK, though,
> which means that if we allow S_DAX to be manipulated via the inode flag we
> can run into this race:
> 
> CPU 0                           CPU 1
> -----                           -----
> xfs_file_write_iter()
>   IS_DAX() << returns false
> 			    xfs_ioctl_setattr()
> 			      xfs_ioctl_setattr_dax_invalidate()
> 			       xfs_ilock(XFS_MMAPLOCK|XFS_IOLOCK)
> 			      sets S_DAX
> 			      releases XFS_MMAPLOCK and XFS_IOLOCK
>   xfs_file_buffered_aio_write()
>   does buffered I/O to DAX inode, death
> 
> Fix this by ensuring that we only check S_DAX when we hold the XFS_IOLOCK
> in the write path.

NACK. This breaks concurrent direct IO write semantics. We must not
take XFS_IOLOCK_EXCL on direct IO writes unless it is absolutely
necessary - there are lots of applications out there that rely on
these semantics for performance.

CHeers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx