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

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Message-ID: <20200603101024.GG19165@quack2.suse.cz>
Date:   Wed, 3 Jun 2020 12:10:24 +0200
From:   Jan Kara <jack@...e.cz>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     ira.weiny@...el.com, linux-kernel@...r.kernel.org,
        linux-xfs@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
        Jan Kara <jack@...e.cz>,
        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 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...

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