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Date:   Fri, 10 Sep 2021 03:11:19 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Pavel Begunkov <asml.silence@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [git pull] iov_iter fixes
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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?

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

I hope so, but I'm afraid that "let's reload from userland on e.g. short
reads" is not better - there's a plenty of interesting corner cases you
need to handle with that.