Re: [RFC][PATCH 00/25] Network fs helper library & fscache kiocb API

David Howells
2021-01-21T20:11:08Z

J. Bruce Fields <bfields@fieldses.org> wrote:

> > J. Bruce Fields <bfields@fieldses.org> wrote:
> >
> > > > Fixing this requires a much bigger overhaul of cachefiles than this patchset
> > > > performs.
> > >
> > > That sounds like "sometimes you may get file corruption and there's
> > > nothing you can do about it". But I know people actually use fscache,
> > > so it must be reliable at least for some use cases.
> >
> > Yes. That's true for the upstream code because that uses bmap.
>
> Sorry, when you say "that's true", what part are you referring to?

Sometimes, theoretically, you may get file corruption due to this.

> > I'm switching
> > to use SEEK_HOLE/SEEK_DATA to get rid of the bmap usage, but it doesn't change
> > the issue.
> >
> > > Is it that those "bridging" blocks only show up in certain corner cases
> > > that users can arrange to avoid? Or that it's OK as long as you use
> > > certain specific file systems whose behavior goes beyond what's
> > > technically required by the bamp or seek interfaces?
> >
> > That's a question for the xfs, ext4 and btrfs maintainers, and may vary
> > between kernel versions and fsck or filesystem packing utility versions.
>
> So, I'm still confused: there must be some case where we know fscache
> actually works reliably and doesn't corrupt your data, right?

Using ext2/3, for example. I don't know under what circumstances xfs, ext4
and btrfs might insert/remove blocks of zeros, but I'm told it can happen.

David