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

[Ira Weiny]

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@xxxxxxxxx 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@xxxxxxxxxxxxx