NAK: Re: [SRU][UNSTABLE/FOCAL/BIONIC][PATCH] UBUNTU: SAUCE: shiftfs: fix dentry revalidation

[Christian Brauner]

On Mon, May 18, 2020 at 08:14:14PM +0200, Christian Brauner wrote:
> BugLink: https://bugs.launchpad.net/bugs/1879196
> 
> While fixing [1] we introduced a regression and triggered [2] whereby
> the underlay and overlay go out of sync because they disagree about the
> dcache. Such situation easily arise when the underlay is modified
> directly. This means the overlay needs to revalidate the dentry in the
> dcache to catchup with the underlay.
> 
> Note, in general it's not advisable to directly modify the underlay
> while a shiftfs mount is on top. In some way this means we need to keep
> two caches in sync and it's hard enough to keep a single cache happy.
> But shiftfs' use-case is inherently prone to be used for exactly that.
> So this is something we have to navigate carefully and honestly we have
> no full model upstream that does the same. Overlayfs has the copy-up
> behavior which let's it get around most of the issues but we don't have
> it and ecryptfs is broken in such scenarios which we verified quite a
> while back.
> In any case, I built a kernel with this patch and re-ran all regressions
> that are related to this that we have so far (cf. [1], [2], and [3]).
> None of them were reproducible with this patch here. So we still fix the
> ESTALE issue but also keep underlay and overlay in sync.
> 
> [1]: https://bugs.launchpad.net/bugs/1872757
> [2]: https://bugs.launchpad.net/bugs/1879196
> [3]: https://bugs.launchpad.net/bugs/1860041
> Cc: Seth Forshee <seth.forshee at canonical.com>
> Cc: James Troup <james.troup at canonical.com>
> Signed-off-by: Christian Brauner <christian.brauner at ubuntu.com>

Sorry, this was the unsquashed version. I'll resend now.

Thanks!
Christian