Re: [RFC PATCH 3/3] overlay: Add the ability to remount volatile directories when safe

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Nov 18, 2020 at 09:24:04AM +0200, Amir Goldstein wrote:
> On Tue, Nov 17, 2020 at 8:29 PM Vivek Goyal <vgoyal@redhat.com> wrote:
> >
> > On Tue, Nov 17, 2020 at 08:03:16PM +0200, Amir Goldstein wrote:
> > > > > C. "shutdown" the filesystem if writeback errors happened and return
> > > > >      EIO from any read, like some blockdev filesystems will do in face
> > > > >      of metadata write errors
> > > > >
> > > > > I happen to have a branch ready for that ;-)
> > > > > https://github.com/amir73il/linux/commits/ovl-shutdown
> > > >
> > > >
> > > > This branch seems to implement shutdown ioctl. So it will still need
> > > > glue code to detect writeback failure in upper/ and trigger shutdown
> > > > internally?
> > > >
> > >
> > > Yes.
> > > ovl_get_acess() can check both the administrative ofs->goingdown
> > > command and the upper writeback error condition for volatile ovl
> > > or something like that.
> >
> > This approach will not help mmaped() pages though, if I do.
> >
> > - Store to addr
> > - msync
> > - Load from addr
> >
> > There is a chance that I can still read back old data.
> >
> 
> msync does not go through overlay. It goes directly to upper fs,
> so it will sync pages and return error on volatile overlay as well.

Ok. Its because vma->vm_file points to realfile.

So even for volatile containers we only avoid fsync/syncfs and not msync.
msync will directly call into upper/. 

> 
> Maybe there will still be weird corner cases, but the shutdown approach
> should cover most or all of the interesting cases.

Agreed.

Vivek