On Wed, Feb 17, 2016 at 02:50:37PM -0700, Ross Zwisler wrote: > On Tue, Feb 16, 2016 at 08:34:16PM -0700, Ross Zwisler wrote: > > Online defrag operations for ext4 are hard coded to use the page cache. > > See ext4_ioctl() -> ext4_move_extents() -> move_extent_per_page() > > > > When combined with DAX I/O, which circumvents the page cache, this can > > result in data corruption. This was observed with xfstests ext4/307 and > > ext4/308. > > > > Fix this by only allowing online defrag for non-DAX files. > > Jan, > > Thinking about this a bit more, it's probably the case that the data > corruption I was observing was due to us skipping the writeback of the dirty > page cache pages because S_DAX was set. > > I do think we have a problem with defrag because it is doing the extent > swapping using the page cache, and we won't flush the dirty pages due to > S_DAX being set. > > This patch is the quick and easy answer, and is perhaps appropriate for v4.5. > > Looking forward, though, what do you think the correct solution is? Making an > extent swapper that doesn't use the page cache (as I believe XFS has? see > xfs_swap_extents()), XFS does the data copy in userspace using direct IO so we don't care about whether DAX is enabled or not on either the source or destination inode. i.e. xfs_swap_extents() is a pure metadata operation, swapping the entire extent tree between two inodes if the source data has not changed while the copy was in progress. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
