On Mon, Jun 18, 2018 at 5:56 PM, James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote: [...] >> > > - Does not break inotify >> > >> > I don't expect it does, but I haven't checked. >> >> I haven't checked either; I'm planning to do so soon. This is a >> concern that was expressed to me by others, I think because inotify >> doesn't work with overlayfs. > > I think shiftfs does work simply because it doesn't really do overlays, > so lots of stuff that doesn't work with overlays does work with it. > I'm afraid shiftfs suffers from the same problems that the old naiive overlayfs inode implementation suffered from. This problem is demonstrated with LTP tests inotify08 inotify09. shiftfs_new_inode() is called on every lookup, so inotify watch may be set on an inode object, then dentry is evicted from cache and then all events on new dentry are not reported on the watched inode. You will need to implement hashed inodes to solve it. Can be done as overlay does - hashing by real inode pointer. This is just one of those subtle things about stacked fs and there may be other in present and more in future - if we don't have a shared code base for the two stacked fs, I wager you are going to end up "cherry picking" fixes often. IMO, an important question to ask is, since both shiftfs and overlayfs are strongly coupled with container use cases, are there users that are interested in both layering AND shifting? on the same "mark"? If the answer is yes, then this may be an argument in favor of integrating at least some of shittfs functionality into overlayfs. Another argument is that shiftfs itself takes the maximum allowed 2 levels of s_stack_depth for it's 2 mounts, so it is actually not possible with current VFS limitation to combine shiftfs with overlayfs. This could be solved relatively easily by adding "-o mark" support to overlayfs and allowing to mount shiftfs also over "marked" overlayfs inside container. Anyway, I'm just playing devil's advocate to the idea of two stacked fs implementation, so presenting this point of view. I am fully aware that there are also plenty of disadvantages to couple two unrelated functionalities together. Cheers, Amir.
