Re: [PATCH 5/5] fs/xfs: Allow toggle of physical DAX flag

Ira Weiny
2019-10-21T22:49:34Z

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