On Wed, Nov 29, 2017 at 8:13 AM, Theodore Ts'o <tytso@xxxxxxx> wrote: > On Tue, Nov 28, 2017 at 03:27:47PM -0600, Ashlie Martinez wrote: >> >> Unfortunately this timing bug only reproduces on some machines. Xiao >> and I have been unable to reproduce this bug (I've tried kvm-xfstests, >> my own kvm VMs, VMs without kvm, VMs with/without virtio drivers, and >> another bare metal system). generic/456 basically sets up a race >> condition between a kernel flusher thread and triggering dm-flakey, so >> I think things like system load, core count, etc. might cause >> different test results. > > Hmm, now I remember the details. It reproduced reliably on > gce-xfstests, but I was able to use kvm-xfstests to debug the problem > (by invocations of debugfs to dump the file system state as I had > described). That's because debugfs operates on the buffer cache, and > before the jbd2 commit, the changes to the inode structure are in the > buffer cache, but they aren't allowed to be persisted on disk until > after the journal commit. And I was using debugfs to dump the inode's > extent tree (as it exists in the buffer cache) before triggering > dm-flakey. > > Now that we understand what is happening, it should be simple to > adjust the test so it reliably reproduces, by adding a "sleep 6" > before _flakey_drop_and_remote. Since the delayed allocation write > won't get resolved until 30 seconds after the inode was first dirtied, > and the default jbd2 timer value is 5 seconds, this should guarantee > that the jbd2 commit has taken place so that the inode changes made by > fallocate are persisted onto the journal, while still allowing the > delayed allocation write to be remain unresolved. > Sorry, sleep 6 didn't work for me. Must be some other subtle detail. If you could work out how to fix the test to catch the bug in kvm-xfstests that would be nice. Better yet, if you can figure out how to configure kvm-xfstests differently so it catches the bug without modifying g/456 that would be much better, because I currently cannot use kvm-xfstest to debug ANY of the dm-log-writes crash test dummies. Thanks, Amir.
