Comment 3 for bug 1879196

I have a fix for this note, that this is a regression we introduced by another fix. I also want to put this cautious note here so people better understand why shiftfs has such bugs and why they are not simple idiot regressions but rather intricate to fix:

    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.