> > My other thought is that perhaps sb_start_write() should invoke > > s_ops->start_write() so that overlay can do the freeze protection on > > the upper early. > > So my understanding of overlayfs is pretty basic so I'm sorry if I miss > something. If I'm right, we have three superblocks here: ovl, upper, lower. > Now 'lower' is read-only so for freezing purposes we can just forget about > it. 'upper' is where the real changes are going into and 'ovl' is a wrapper > virtual superblock that handles merging of 'lower' and 'upper'. Correct so > far? Yes. > > And the problem seems to be that when you acquire freeze protection for the > 'ovl' superblock, you in fact want to acquire freeze protection for the > 'upper' (as 'ovl' is just virtual and has no disk state to protect). So I There are use case for freezing ovl (i.e. ovl snapshots) but it is not implemented at the moment. Overlayfs already gets upper freeze protection internally before any modification to upper. The problem that locking order of upper freeze is currently under overlay inode mutex. And that brings a problem with the above pipe case. > agree that a callback to allow overlayfs to acquire freeze protection on > 'upper' right away would be one solution. Or we could make s_writers a > pointer and redirect ovl->s_writers to upper->s_writers. Then VFS should do > the right thing from the start unless overlayfs calls back into operations > on 'upper' that will try to acquire the freeze protection again. Thoughts? Overlayfs definitely calls into operations on upper and upper certainly acquires several levels of s_writers itself. The problem with the proposal to change locking order to ovl freeze -> upper freeze -> ovl inode -> upper inode is that for some non-write operations (e.g. lookup, readdir) overlay may end up updating xattrs on upper, so will need to take upper freeze after ovl inode lock without ovl freeze being called by vfs. I suggested that we may use upper freeze trylock in those cases and skip xattr update if trylock fails. Not sure if my assumption is correct that this would be ok w.r.t locking rules? Not sure if we can get away with trylock in all the cases that we need to modify upper. Thanks, Amir.
