On 11/18/2014 10:45 PM, Dave Chinner wrote:
On Fri, Nov 14, 2014 at 04:16:30PM -0500, Josef Bacik wrote:
We use the modified list to keep track of which extents have been modified so we
know which ones are candidates for logging at fsync() time. Newly modified
extents are added to the list at modification time, around the same time the
ordered extent is created. We do this so that we don't have to wait for ordered
extents to complete before we know what we need to log. The problem is when
something like this happens
log extent 0-4k on inode 1
copy csum for 0-4k from ordered extent into log
sync log
commit transaction
log some other extent on inode 1
ordered extent for 0-4k completes and adds itself onto modified list again
log changed extents
see ordered extent for 0-4k has already been logged
at this point we assume the csum has been copied
sync log
crash
On replay we will see the extent 0-4k in the log, drop the original 0-4k extent
which is the same one that we are replaying which also drops the csum, and then
we won't find the csum in the log for that bytenr. This of course causes us to
have errors about not having csums for certain ranges of our inode. So remove
the modified list manipulation in unpin_extent_cache, any modified extents
should have been added well before now, and we don't want them re-logged. This
fixes my test that I could reliably reproduce this problem with. Thanks,
Is it possiible to turn this unspecified test in into another
generic fsync xfstest?
It depends on a new dm target I'm working on to better test power fail
scenarios, once I have that merged I have a few xfstests I'll be
submitting in this area. Would you actually mind taking a quick look at
it to make sure it seems sane?
https://git.kernel.org/cgit/linux/kernel/git/josef/btrfs-next.git/log/?h=dm-powerfail
The 'split' option is what is meant for ext* and xfs (I haven't tested
that part yet), which will just return the old data in the case of
unflushed data/metadata. Anything you'd like to see added or changed?
Thanks,
Josef
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