Re: [RFC][PATCH v2 0/5] Experiments with overlayfs filemap

Miklos Szeredi
2019-01-28T21:17:31Z

On Mon, Jan 28, 2019 at 9:58 PM Amir Goldstein <amir73il@gmail.com> wrote:
>
> > > > I'll stick with the simple copy_page() implementation (back and forth)
> > > > unless you come up with a better suggestion. We will probably want
> > > > something more efficient later on, but I rather start with something
> > > > simple and working.
> > > >
> > >
> > > Like this:
> > > https://github.com/amir73il/linux/commits/ovl-aops-wip
> > >
> >
> > Am I understanding it right, that we will use page cache at overlay inode
> > as well as upper file system. So there is duplication and memory usage
> > for upper files will double?
>
> That won't be acceptable, but using upper page cache as temporary pages
> for the duration of writeback makes overlayfs code much simpler IMO and
> avoids vma -> vfs locking order issues.
>
> My idea was to get rid of the upper pages once writeback is done
> (or periodically), but that wasn't implemented yet, because I am still
> waiting to hear if the idea is inherently bad.

The problem with that is unnecessary memory copies. If done right,
direct I/O will be simpler, faster, and won't have locking issues,
beacuse it can be done asynchronously.

Trying to put some code together...

Thanks,
Miklos