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 [thread overview]
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
next prev parent reply other threads:[~2019-11-08 13:46 UTC|newest]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20191108134606.GL20863@quack2.suse.cz \
--to=jack@suse.cz \
--cc=dan.j.williams@intel.com \
--cc=darrick.wong@oracle.com \
--cc=david@fromorbit.com \
--cc=hch@lst.de \
--cc=ira.weiny@intel.com \
--cc=linux-ext4@vger.kernel.org \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-xfs@vger.kernel.org \
--cc=tytso@mit.edu \
--cc=viro@zeniv.linux.org.uk \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).