On Tue, Feb 12, 2019 at 2:39 PM Amir Goldstein <amir73il@xxxxxxxxx> wrote: > > > > 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. sb_start_write() calls cannot be nested (for the same reason two shared locks may be part of a deadlock), so this would mean having to make sure that none of the code that overlay calls does sb_start_write() itself. Which means quite a bit of vfs api churn and the associated pain... > 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? Yes, using trylock is always deadlock free. Thanks, Miklos
