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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>
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From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <f02eae7c-f636-c057-4140-2e688393f79d@kernel.dk>
Date:   Thu, 9 Sep 2021 21:30:03 -0600
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On 9/9/21 9:27 PM, Al Viro wrote:
> 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.

Yes that was wrong, the iovec is obviously const. But that really
doesn't change the original point, which was that copying the iov_iter
itself unconditionally would be miserable.

-- 
Jens Axboe