Re: [PATCH v3 05/12] fs: remove unneeded IS_DAX() check

Ira Weiny
2020-02-12T16:04:43Z

On Wed, Feb 12, 2020 at 07:41:07AM +1100, Dave Chinner wrote:
> On Tue, Feb 11, 2020 at 08:38:31AM -0800, Ira Weiny wrote:
> > On Tue, Feb 11, 2020 at 04:34:01PM +1100, Dave Chinner wrote:
> > > On Sat, Feb 08, 2020 at 11:34:38AM -0800, ira.weiny@intel.com wrote:
> > > > From: Ira Weiny <ira.weiny@intel.com>
> > > >
> > > > The IS_DAX() check in io_is_direct() causes a race between changing the
> > > > DAX state and creating the iocb flags.
> > > >
> > > > Remove the check because DAX now emulates the page cache API and
> > > > therefore it does not matter if the file state is DAX or not when the
> > > > iocb flags are created.
> > >
> > > This statement is ... weird.
> > >
> > > DAX doesn't "emulate" the page cache API at all
> >
> > ah... yea emulate is a bad word here.
> >
> > > - it has it's own
> > > read/write methods that filesystems call based on the iomap
> > > infrastructure (dax_iomap_rw()). i.e. there are 3 different IO paths
> > > through the filesystems: the DAX IO path, the direct IO path, and
> > > the buffered IO path.
> > >
> > > Indeed, it seems like this works a bit by luck: Ext4 and XFS always
> > > check IS_DAX(inode) in the read/write_iter methods before checking
> > > for IOCB_DIRECT, and hence the IOCB_DIRECT flag is ignored by the
> > > filesystems. i.e. when we got rid of the O_DIRECT paths from DAX, we
> > > forgot to clean up io_is_direct() and it's only due to the ordering
> > > of checks that we went down the DAX path correctly....
> > >
> > > That said, the code change is good, but the commit message needs a
> > > rewrite.
> >
> > How about?
> >
> > <commit msg>
> > fs: Remove unneeded IS_DAX() check
> >
> > The IS_DAX() check in io_is_direct() causes a race between changing the
> > DAX state and creating the iocb flags.
>
> This is irrelevant - the check is simply wrong and has been since
> ~2016 when we moved DAX to use the iomap infrastructure...

Deleted.
Ira

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