Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [git pull] iov_iter fixes
To:     Al Viro <viro@zeniv.linux.org.uk>
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>
References: <YTmL/plKyujwhoaR@zeniv-ca.linux.org.uk>
 <CAHk-=wiacKV4Gh-MYjteU0LwNBSGpWrK-Ov25HdqB1ewinrFPg@mail.gmail.com>
 <5971af96-78b7-8304-3e25-00dc2da3c538@kernel.dk>
 <YTrJsrXPbu1jXKDZ@zeniv-ca.linux.org.uk>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <b8786a7e-5616-ce83-c2f2-53a4754bf5a4@kernel.dk>
Date:   Thu, 9 Sep 2021 21:05:13 -0600
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.10.0
MIME-Version: 1.0
In-Reply-To: <YTrJsrXPbu1jXKDZ@zeniv-ca.linux.org.uk>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk

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.

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

-- 
Jens Axboe