Re: [PATCH] ovl: make private mounts longterm

Miklos Szeredi
2020-05-22T18:54:06Z

On Fri, May 22, 2020 at 7:02 PM Amir Goldstein <amir73il@gmail.com> wrote:
>
> > > > - mntput(ofs->upper_mnt);
> > > > - for (i = 1; i < ofs->numlayer; i++) {
> > > > - iput(ofs->layers[i].trap);
> > > > - mntput(ofs->layers[i].mnt);
> > > > +
> > > > + if (!ofs->layers) {
> > > > + /* Deal with partial setup */
> > > > + kern_unmount(ofs->upper_mnt);
> > > > + } else {
> > > > + /* Hack! Reuse ofs->layers as a mounts array */
> > > > + struct vfsmount **mounts = (struct vfsmount **) ofs->layers;
> > > > +
> > > > + for (i = 0; i < ofs->numlayer; i++) {
> > > > + iput(ofs->layers[i].trap);
> > > > + mounts[i] = ofs->layers[i].mnt;
> > > > + }
> > > > + kern_unmount_many(mounts, ofs->numlayer);
> > > > + kfree(ofs->layers);
> > >
> > > That's _way_ too subtle. AFAICS, you rely upon ->upper_mnt == ->layers[0].mnt,
> > > ->layers[0].trap == NULL, without even mentioning that. And the hack you do
> > > mention... Yecchhh... How many layers are possible, again?
> >
> > 500, mounts array would fit inside a page and a page can be allocated
> > with __GFP_NOFAIL. But why bother? It's not all that bad, is it?
>
> FWIW, it seems fine to me.
> We can transfer the reference from upperdir_trap to layers[0].trap
> when initializing layers[0] for the sake of clarity.

Right, we should just get rid of ofs->upper_mnt and ofs->upperdir_trap
and use ofs->layers[0] to store those.

Thanks,
Miklos