On 9/9/21 8:57 PM, Al Viro wrote:

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

quoted

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.

It's 48 bytes, and we have to do it upfront. That means we'd be doing it
for _all_ requests, not just when we need to retry. As an example, current
benchmarks are at ~4M read requests per core. That'd add ~200MB/sec of
memory traffic just doing this copy.

Besides, I think that's moot as there's a better way.

-- 
Jens Axboe