On Wed, Feb 27, 2019 at 7:01 AM Qu Wenruo <wqu@xxxxxxx> wrote: > > > > On 2019/2/27 下午12:49, Amir Goldstein wrote: > [snip] > >>> Indeed. > >> > >> May I ask a stupid question? > >> > >> How does it matter whether the device is clean or not? > >> Shouldn't the journal/metadata or whatever be self-contained? > >> > > > > Yes and no. > > > > The most simple example (not limited to xfs and not sure it is like that in xfs) > > is how you find the last valid journal commit entry. It should have correct CRC > > and the largest LSN. But it you replay IO on top of existing journal without > > wiping it first, then journal recovery will continue past the point to meant to > > replay or worse. > > The journal doesn't have some superblock like system to tell where to > stop replaying? > Yes, it does. My statement was inaccurate. The point is how can filesystem trust that journal superblock is not corrupted? If filesystem observes data from the future in the journal that is not expected to be there, or any checksumed metadata on the device, it may fail sanity checks that superblock or other metadata are not corrupted. Suppose filesystem has a bug that overwrites new metadata from time N with old stale metadata buffer from time N-1. That would look not much different than log-writes replay to time N-1 over non clean device from time N. > If not, then indeed we need to discard the journal before writing new one. > ... > >> Am I missing something? Or do I get too poisoned by btrfs CoW? > >> > > > > I'd be very surprised if btrfs cannot be flipped by seeing stale data "from > > the future" in the block device. Seems to me like the entire concept of > > CoW and metadata checksums is completely subverted by the existence > > of correct checksums on "stale metadata from the future". > > It seems that metadata CoW makes it impossible to see future data. > > All btree trees get updated CoW, so no metadata will be overwritten > during one transaction. > Only super block is overwritten and normally superblock is updated > atomically. > > So either old superblock is still here, all we can see is old tree pointers. > Or new superblock is here, all we can see is new tree pointers. > And new metadata will never be written into old metadata, there is no > way to see future metadata. > Those assumptions could fail if you have unreliable hardware that reorders IO across FUA, just drops IO on the floor or a bug in the filesystem or block layer. Existence of metadata from the future could look like any of the above has happened. Thanks, Amir.
