lkml.org 
[lkml]   [2017]   [Sep]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 4/7] xfs: protect S_DAX transitions in XFS write path
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@fromorbit.com

\
 
 \ /
  Last update: 2017-09-26 02:10    [W:0.130 / U:0.172 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site