linux-kernel - Re: [PATCH V11 11/11] fs/xfs: Update xfs_ioctl_setattr_dax

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Date:   Wed, 3 Jun 2020 10:03:44 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Jan Kara <jack@...e.cz>
Cc:     ira.weiny@...el.com, linux-kernel@...r.kernel.org,
        linux-xfs@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jeff Moyer <jmoyer@...hat.com>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-api@...r.kernel.org
Subject: Re: [PATCH V11 11/11] fs/xfs: Update
 xfs_ioctl_setattr_dax_invalidate()

On Wed, Jun 03, 2020 at 12:10:24PM +0200, Jan Kara wrote:
> On Tue 02-06-20 10:23:53, Darrick J. Wong wrote:
> > On Tue, Apr 28, 2020 at 01:11:38PM -0700, Darrick J. Wong wrote:
> > > > -out_unlock:
> > > > -	xfs_iunlock(ip, XFS_MMAPLOCK_EXCL | XFS_IOLOCK_EXCL);
> > > > -	return error;
> > > > +	if ((mp->m_flags & XFS_MOUNT_DAX_ALWAYS) ||
> > > > +	    (mp->m_flags & XFS_MOUNT_DAX_NEVER))
> > > > +		return;
> > > >  
> > > > +	if (((fa->fsx_xflags & FS_XFLAG_DAX) &&
> > > > +	    !(ip->i_d.di_flags2 & XFS_DIFLAG2_DAX)) ||
> > > > +	    (!(fa->fsx_xflags & FS_XFLAG_DAX) &&
> > > > +	     (ip->i_d.di_flags2 & XFS_DIFLAG2_DAX)))
> > > > +		d_mark_dontcache(inode);
> > 
> > Now that I think about this further, are we /really/ sure that we want
> > to let unprivileged userspace cause inode evictions?
> 
> You have to have an equivalent of write access to the file to be able to
> trigger d_mark_dontcache(). So you can e.g. delete it.  Or you could
> fadvise / madvise regarding its page cache. I don't see the ability to push
> inode out of cache as stronger than the abilities you already have...

<nod> Ok.  I just had one last bout of paranoia, but I think it'll be
fine. :)

--D

> 								Honza
> -- 
> Jan Kara <jack@...e.com>
> SUSE Labs, CR