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From: Jan Kara <jack@suse.cz>
To: Ira Weiny <ira.weiny@intel.com>
Cc: Dave Chinner <david@fromorbit.com>,
	linux-kernel@vger.kernel.org,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	"Darrick J. Wong" <darrick.wong@oracle.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Christoph Hellwig <hch@lst.de>,
	"Theodore Y. Ts'o" <tytso@mit.edu>, Jan Kara <jack@suse.cz>,
	linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 5/5] fs/xfs: Allow toggle of physical DAX flag
Date: Fri, 8 Nov 2019 14:46:06 +0100
Message-ID: <20191108134606.GL20863@quack2.suse.cz> (raw)
In-Reply-To: <20191108131238.GK20863@quack2.suse.cz>

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@intel.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@suse.com>
SUSE Labs, CR

  reply index

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-20 15:59 [PATCH 0/5] Enable per-file/directory DAX operations ira.weiny
2019-10-20 15:59 ` [PATCH 1/5] fs/stat: Define DAX statx attribute ira.weiny
2019-10-22 11:32   ` Boaz Harrosh
2019-10-22 16:51     ` Ira Weiny
2019-10-20 15:59 ` [PATCH 2/5] fs/xfs: Isolate the physical DAX flag from effective ira.weiny
2019-10-21  0:26   ` Dave Chinner
2019-10-21 17:40     ` Ira Weiny
2019-10-20 15:59 ` [PATCH 3/5] fs/xfs: Separate functionality of xfs_inode_supports_dax() ira.weiny
2019-10-20 15:59 ` [PATCH 4/5] fs/xfs: Clean up DAX support check ira.weiny
2019-10-20 15:59 ` [PATCH 5/5] fs/xfs: Allow toggle of physical DAX flag ira.weiny
2019-10-21  0:45   ` Dave Chinner
2019-10-21 22:49     ` Ira Weiny
2019-10-21 23:46       ` Dave Chinner
2019-11-08 13:12       ` Jan Kara
2019-11-08 13:46         ` Jan Kara [this message]
2019-11-08 19:36           ` Ira Weiny
2019-11-11 16:07             ` Jan Kara
2019-11-11 23:54               ` Ira Weiny
2019-10-22 11:21 ` [PATCH 0/5] Enable per-file/directory DAX operations Boaz Harrosh
2019-10-23 13:09   ` Boaz Harrosh
2019-10-23 22:13     ` Dave Chinner
2019-10-24  2:31       ` Boaz Harrosh
2019-10-24  7:34         ` Dave Chinner
2019-10-24 14:05           ` Boaz Harrosh
2019-10-24 21:35             ` Dave Chinner
2019-10-24 23:29               ` Boaz Harrosh
2019-10-25  0:36                 ` Dave Chinner
2019-10-25  1:15                   ` Boaz Harrosh
2019-10-25 20:49                   ` Ira Weiny
2019-10-27 22:10                     ` Dave Chinner
2019-10-31 16:17                       ` Ira Weiny
2019-11-01 22:47                         ` Dave Chinner
2019-11-02  4:25                           ` Dan Williams

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