On Thu, Sep 09, 2021 at 03:19:56PM -0600, Jens Axboe wrote:

Not sure how we'd do that, outside of stupid tricks like copy the
iov_iter before we pass it down. But that's obviously not going to be
very efficient. Hence we're left with having some way to reset/reexpand,
even in the presence of someone having done truncate on it.

"Obviously" why, exactly?  It's not that large a structure; it's not
the optimal variant, but I'd like to see profiling data before assuming
that it'll cause noticable slowdowns.