Re: [PATCH 15/39] ovl: add helper to return real file

From: Miklos Szeredi
Date: Mon Jun 11 2018 - 04:11:48 EST

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...