On Sun, Jun 10, 2018 at 7:42 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: > On Tue, May 29, 2018 at 04:43:15PM +0200, Miklos Szeredi wrote: >> In the common case we can just use the real file cached in >> file->private_data. There are two exceptions: >> >> 1) File has been copied up since open: in this unlikely corner case just >> use a throwaway real file for the operation. If ever this becomes a >> perfomance problem (very unlikely, since overlayfs has been doing most fine >> without correctly handling this case at all), then we can deal with that by >> updating the cached real file. > > See the ovl_mmap() problem. FWIW, I would probably suggest something along > the lines of > ->private_data either points to struct file, or is 1 | address of > 2-element array of struct file * > odd value => mask bit 0 away, cast to struct file ** and dereference > even value and it's still in the right layer => use that > even value and it is in the wrong layer => > allocate a two-pointer array > open in the right layer > stick that into array[0] and original - into array[1] > cmpxchg array | 1 into ->private_data > if that succeeds > return array[0] > else > fput array[0], free array, then use the value returned > by cmpxchg - mask bit 0 away, cast and dereference Iff we really need that complexity, then yes, that's a nice solution. But I think we don't: see incremental posted for ->mmap() issue + for plain I/O we don't really care about this case, since it happens so rarely. Maybe later... Thanks, Miklos
