On Thu, Apr 09, 2020 at 07:09:50AM +1000, Dave Chinner wrote: > On Wed, Apr 08, 2020 at 11:58:03AM +0200, Jan Kara wrote: > > On Wed 08-04-20 12:23:18, Dave Chinner wrote: > > > On Tue, Apr 07, 2020 at 11:29:57AM -0700, ira.weiny@xxxxxxxxx wrote: > > > > From: Ira Weiny <ira.weiny@xxxxxxxxx> > > > > > > > > We only support changing FS_XFLAG_DAX on directories. Files get their > > > > flag from the parent directory on creation only. So no data > > > > invalidation needs to happen. > > > > > > Which leads me to ask: how are users and/or admins supposed to > > > remove the flag from regular files once it is set in the filesystem? > > > > > > Only being able to override the flag via the "dax=never" mount > > > option means that once the flag is set, nobody can ever remove it > > > and they can only globally turn off dax if it gets set incorrectly. > > > It also means a global interrupt because all apps on the filesystem > > > need to be stopped so the filesystem can be unmounted and mounted > > > again with dax=never. This is highly unfriendly to admins and users. > > > > > > IOWs, we _must_ be able to clear this inode flag on regular inodes > > > in some way. I don't care if it doesn't change the current in-memory > > > state, but we must be able to clear the flags so that the next time > > > the inodes are instantiated DAX is not enabled for those files... > > > > Well, there's one way to clear the flag: delete the file. If you still care > > about the data, you can copy the data first. It isn't very convenient, I > > agree, and effectively means restarting whatever application that is using > > the file. > > Restarting the application is fine. Having to backup/restore or copy > the entire data set just to turn off an inode flag? That's not a > viable management strategy. We could be talking about terabytes of > data here. > > I explained how we can safely remove the flag in the other branch of > this thread... > > > But it seems like more understandable API than letting user clear > > the on-disk flag but the inode will still use DAX until kernel decides to > > evict the inode > > Certainly doesn't seem that way to me. "stop app, clear flags, drop > caches, restart app" is a pretty simple, easy thing to do for an > admin. I want to be clear here: I think this is reasonable. However, I don't see consensus for that interface. Christoph in particular said that a 'lazy change' is: "... straight from the playbook for arcane and confusing API designs." "But returning an error and doing a lazy change anyway is straight from the playbook for arcane and confusing API designs." -- https://lore.kernel.org/lkml/20200403072731.GA24176@xxxxxx/ Did I somehow misunderstand this? Again for this patch set, 5.8, lets leave that alone for now. I think if we disable setting this on files right now we can still allow it in the future as another step forward. > > Especially compared to process that is effectively "stop app, backup > data set, delete data set, clear flags, restore data set, restart > app" > > > - because that often means you need to restart the > > application using the file anyway for the flag change to have any effect. > > That's a trivial requirement compared to the downtime and resource > cost of a data set backup/restore just to clear inode flags.... > I agree but others do not. This still provides a baby step forward and some granularity for those who plan out the creation of their files. Ira
