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Date:   Fri, 8 Nov 2019 14:46:06 +0100
From:   Jan Kara <jack@...e.cz>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     Dave Chinner <david@...morbit.com>, linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 5/5] fs/xfs: Allow toggle of physical DAX flag

On Fri 08-11-19 14:12:38, Jan Kara wrote:
> On Mon 21-10-19 15:49:31, Ira Weiny wrote:
> > 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@...el.com wrote:
> > > 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.  :-(
> 
> Just as a datapoint, ext4 is bold and sets inode->i_mapping->a_ops on
> existing inodes when switching journal data flag and so far it has not
> blown up. What we did to deal with issues Dave describes is that we
> introduced percpu rw-semaphore guarding switching of aops and then inside
> problematic functions redirect callbacks in the right direction under this
> semaphore. Somewhat ugly but it seems to work.

Thinking about this some more, perhaps this scheme could be actually
transformed in something workable. We could have a global (or maybe per-sb
but I'm not sure it's worth it) percpu rwsem and we could transform aops
calls into:

percpu_down_read(aops_rwsem);
do_call();
percpu_up_read(aops_rwsem);

With some macro magic it needn't be even that ugly.

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

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