Re: [git pull] iov_iter fixes

[Al Viro <viro@zeniv.linux.org.uk>]

On Thu, Sep 09, 2021 at 09:22:30PM -0600, Jens Axboe wrote:
> On 9/9/21 9:11 PM, Al Viro wrote:
> > On Thu, Sep 09, 2021 at 09:05:13PM -0600, Jens Axboe wrote:
> >> On 9/9/21 8:57 PM, Al Viro wrote:
> >>> 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.
> >>
> >> 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.
> > 
> > Umm...  How much of that will be handled by cache?
> 
> Depends? And what if the iovec itself has been modified in the middle?
> We'd need to copy that whole thing too. It's just not workable as a
> solution.

Huh?  Why the hell would we need to copy iovecs themselves?  They are never
modified by ->read_iter()/->write_iter().

That's the whole fucking point of iov_iter - the iovec itself is made
constant, with all movable parts taken to iov_iter.

Again, we should never, ever modify the iovec (or bvec, etc.) array in
->read_iter()/->write_iter()/->sendmsg()/etc. instances.  If you see such
behaviour anywhere, report it immediately.  Any such is a blatant bug.