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
