From: Ira Weiny <ira.weiny@intel.com> To: Dave Chinner <david@fromorbit.com> Cc: 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 v3 06/12] fs/xfs: Check if the inode supports DAX under lock Date: Tue, 11 Feb 2020 09:55:09 -0800 Message-ID: <20200211175509.GD12866@iweiny-DESK2.sc.intel.com> (raw) In-Reply-To: <20200211061639.GH10776@dread.disaster.area> On Tue, Feb 11, 2020 at 05:16:39PM +1100, Dave Chinner wrote: > On Sat, Feb 08, 2020 at 11:34:39AM -0800, ira.weiny@intel.com wrote: > > From: Ira Weiny <ira.weiny@intel.com> > > > > One of the checks for an inode supporting DAX is if the inode is > > reflinked. During a non-DAX to DAX state change we could race with > > the file being reflinked and end up with a reflinked file being in DAX > > state. > > > > Prevent this race by checking for DAX support under the MMAP_LOCK. > > The on disk inode flags are protected by the XFS_ILOCK, not the > MMAP_LOCK. i.e. the MMAPLOCK provides data access serialisation, not > metadata modification serialisation. Ah... > > > > > Signed-off-by: Ira Weiny <ira.weiny@intel.com> > > --- > > fs/xfs/xfs_ioctl.c | 11 +++++++---- > > 1 file changed, 7 insertions(+), 4 deletions(-) > > > > diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c > > index da1eb2bdb386..4ff402fd6636 100644 > > --- a/fs/xfs/xfs_ioctl.c > > +++ b/fs/xfs/xfs_ioctl.c > > @@ -1194,10 +1194,6 @@ xfs_ioctl_setattr_dax_invalidate( > > > > *join_flags = 0; > > > > - if ((fa->fsx_xflags & FS_XFLAG_DAX) == FS_XFLAG_DAX && > > - !xfs_inode_supports_dax(ip)) > > - return -EINVAL; > > - > > /* If the DAX state is not changing, we have nothing to do here. */ > > if ((fa->fsx_xflags & FS_XFLAG_DAX) && > > (ip->i_d.di_flags2 & XFS_DIFLAG2_DAX)) > > @@ -1211,6 +1207,13 @@ xfs_ioctl_setattr_dax_invalidate( > > > > /* lock, flush and invalidate mapping in preparation for flag change */ > > xfs_ilock(ip, XFS_MMAPLOCK_EXCL | XFS_IOLOCK_EXCL); > > + > > + if ((fa->fsx_xflags & FS_XFLAG_DAX) == FS_XFLAG_DAX && > > + !xfs_inode_supports_dax(ip)) { > > + error = -EINVAL; > > + goto out_unlock; > > + } > > Yes, you might be able to get away with reflink vs dax flag > serialisation on the inode flag modification, but it is not correct and > leaves a landmine for future inode flag modifications that are done > without holding either the MMAP or IOLOCK. > > e.g. concurrent calls to xfs_ioctl_setattr() setting/clearing flags > other than the on disk DAX flag are all serialised by the ILOCK_EXCL > and will no be serialised against this DAX check. Hence if there are > other flags that we add in future that affect the result of > xfs_inode_supports_dax(), this code will not be correctly > serialised. > > This raciness in checking the DAX flags is the reason that > xfs_ioctl_setattr_xflags() redoes all the reflink vs dax checks once > it's called under the XFS_ILOCK_EXCL during the actual change > transaction.... Ok I found this by trying to make sure that the xfs_inode_supports_dax() call was always returning valid data. So I don't have a specific test which was failing. Looking at the code again, it sounds like I was wrong about which locks protect what and with your explanation above it looks like there is nothing to be done here and I can drop the patch. Would you agree? Thanks for the review! Ira > > Cheers, > > Dave. > -- > Dave Chinner > david@fromorbit.com
next prev parent reply index Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-02-08 19:34 [PATCH v3 00/12] Enable per-file/directory DAX operations V3 ira.weiny 2020-02-08 19:34 ` [PATCH v3 01/12] fs/stat: Define DAX statx attribute ira.weiny 2020-02-08 19:34 ` [PATCH v3 02/12] fs/xfs: Isolate the physical DAX flag from effective ira.weiny 2020-02-08 19:34 ` [PATCH v3 03/12] fs/xfs: Separate functionality of xfs_inode_supports_dax() ira.weiny 2020-02-11 5:47 ` Dave Chinner 2020-02-11 16:13 ` Ira Weiny 2020-02-08 19:34 ` [PATCH v3 04/12] fs/xfs: Clean up DAX support check ira.weiny 2020-02-11 5:57 ` Dave Chinner 2020-02-11 16:28 ` Ira Weiny 2020-02-11 20:38 ` Dave Chinner 2020-02-08 19:34 ` [PATCH v3 05/12] fs: remove unneeded IS_DAX() check ira.weiny 2020-02-11 5:34 ` Dave Chinner 2020-02-11 16:38 ` Ira Weiny 2020-02-11 20:41 ` Dave Chinner 2020-02-12 16:04 ` Ira Weiny 2020-02-08 19:34 ` [PATCH v3 06/12] fs/xfs: Check if the inode supports DAX under lock ira.weiny 2020-02-11 6:16 ` Dave Chinner 2020-02-11 17:55 ` Ira Weiny [this message] 2020-02-11 20:42 ` Dave Chinner 2020-02-12 16:10 ` Ira Weiny 2020-02-08 19:34 ` [PATCH v3 07/12] fs: Add locking for a dynamic DAX state ira.weiny 2020-02-11 8:00 ` Dave Chinner 2020-02-11 20:14 ` Ira Weiny 2020-02-11 20:59 ` Dan Williams 2020-02-11 21:49 ` Dave Chinner 2020-02-12 6:31 ` Darrick J. Wong 2020-02-08 19:34 ` [PATCH v3 08/12] fs/xfs: Clarify lockdep dependency for xfs_isilocked() ira.weiny 2020-02-08 19:34 ` [PATCH v3 09/12] fs/xfs: Add write DAX lock to xfs layer ira.weiny 2020-02-08 19:34 ` [PATCH v3 10/12] fs: Prevent DAX state change if file is mmap'ed ira.weiny 2020-02-08 19:34 ` [PATCH v3 11/12] fs/xfs: Clean up locking in dax invalidate ira.weiny 2020-02-08 19:34 ` [PATCH v3 12/12] fs/xfs: Allow toggle of effective DAX flag ira.weiny 2020-02-10 15:15 ` [PATCH v3 00/12] Enable per-file/directory DAX operations V3 Jeff Moyer 2020-02-11 20:17 ` Ira Weiny 2020-02-12 19:49 ` Jeff Moyer 2020-02-13 19:01 ` Ira Weiny 2020-02-13 19:05 ` Ira Weiny 2020-02-13 19:58 ` Darrick J. Wong 2020-02-13 23:29 ` Ira Weiny 2020-02-14 0:16 ` Dan Williams 2020-02-14 20:06 ` Ira Weiny 2020-02-14 21:23 ` Jeff Moyer 2020-02-14 21:58 ` Ira Weiny 2020-02-14 22:06 ` Jeff Moyer 2020-02-14 22:58 ` Jeff Moyer 2020-02-14 23:03 ` Jeff Moyer 2020-02-18 2:35 ` Ira Weiny 2020-02-18 14:22 ` Jeff Moyer 2020-02-18 23:54 ` Ira Weiny 2020-02-20 16:20 ` Ira Weiny 2020-02-20 16:30 ` Darrick J. Wong 2020-02-20 16:49 ` Ira Weiny 2020-02-20 17:00 ` Darrick J. Wong
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