Re: [PATCH] block: Fix S_DAX inode flag locking

[Dave Chinner <david@fromorbit.com>]

On Tue, May 17, 2016 at 12:34:57PM -0700, Dan Williams wrote:
> On Tue, May 17, 2016 at 11:29 AM, Jon Derrick
> <jonathan.derrick@intel.com> wrote:
> > This patch fixes S_DAX bd_inode i_flag locking to conform to suggested
> 
> A "fix" implies that its currently broken.  I don't see how it is, not
> until we add an ioctl method or other path that also tries to update
> the flags outside of blkdev_get() context.  So, I don't think this
> patch stands on its own if you were intending it to be merged
> separately.
> 
> > locking rules. It is presumed that S_DAX is the only valid inode flag
> > for a block device which subscribes to direct-access, and will restore
> > any previously set flags if direct-access initialization fails.
> >
> > This reverts to i_flags behavior prior to
> > bbab37ddc20bae4709bca8745c128c4f46fe63c5
> > by allowing other bd_inode flags when DAX is disabled
> >
> > Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
> > ---
> >  fs/block_dev.c | 31 ++++++++++++++++++++++++++-----
> >  1 file changed, 26 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/block_dev.c b/fs/block_dev.c
> > index 20a2c02..d41e37f 100644
> > --- a/fs/block_dev.c
> > +++ b/fs/block_dev.c
> > @@ -1159,6 +1159,20 @@ void bd_set_size(struct block_device *bdev, loff_t size)
> >  }
> >  EXPORT_SYMBOL(bd_set_size);
> >
> > +static void bd_add_dax(struct inode *inode)
> > +{
> > +       inode_lock(inode);
> > +       inode->i_flags |= S_DAX;
> > +       inode_unlock(inode);
> > +}
> > +
> > +static void bd_clear_dax(struct inode *inode)
> > +{
> > +       inode_lock(inode);
> > +       inode->i_flags &= ~S_DAX;
> > +       inode_unlock(inode);
> > +}
> 
> Since this is inode generic should these helpers be prefixed "i_"
> rather than "bd_"?

Probably not, because in general filesystems are responsible for
updating i_flags to reflect on-disk inode configuration and that's
typically done under transaction contexts. e.g.  through ioctl
interfaces to set/clear flags that are stored on disk.  As such,
inode->i_flags is effectively protected by the filesystem specific
locking heirarchy, not the generic inode_lock().

e.g. have a look at XFS storing a persistent "DAX-enabled" flag in
the inode, which can be set/cleared on individual inodes dynamically
by FS_IOC_FSSETXATTR. The XFS i_flags update function assumes
exclusive access to the field as it is called under locked
transaction context. Similar code exists in ext4, btrfs, gfs2,
etc....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com