From: Jens Axboe <axboe@kernel.dk>
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>
Subject: Re: [git pull] iov_iter fixes
Date: Thu, 9 Sep 2021 21:30:03 -0600 [thread overview]
Message-ID: <f02eae7c-f636-c057-4140-2e688393f79d@kernel.dk> (raw)
In-Reply-To: <YTrQuvqvJHd9IObe@zeniv-ca.linux.org.uk>
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
next prev parent reply other threads:[~2021-09-10 3:30 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-09 4:22 [git pull] iov_iter fixes Al Viro
2021-09-09 19:37 ` Linus Torvalds
2021-09-09 21:19 ` Jens Axboe
2021-09-09 21:39 ` Jens Axboe
2021-09-09 21:56 ` Linus Torvalds
2021-09-09 22:21 ` Jens Axboe
2021-09-09 22:56 ` Linus Torvalds
2021-09-10 1:35 ` Jens Axboe
2021-09-10 2:43 ` Jens Axboe
2021-09-10 2:48 ` Al Viro
2021-09-10 3:06 ` Jens Axboe
2021-09-10 3:15 ` Al Viro
2021-09-10 3:23 ` Jens Axboe
2021-09-10 3:24 ` Al Viro
2021-09-10 3:28 ` Jens Axboe
2021-09-13 15:29 ` David Laight
2021-09-09 21:42 ` Dave Chinner
2021-09-10 2:57 ` Al Viro
2021-09-10 3:05 ` Jens Axboe
2021-09-10 3:11 ` Al Viro
2021-09-10 3:22 ` Jens Axboe
2021-09-10 3:27 ` Al Viro
2021-09-10 3:30 ` Jens Axboe [this message]
2021-09-10 3:36 ` Al Viro
2021-09-10 13:57 ` Jens Axboe
2021-09-10 14:42 ` Al Viro
2021-09-10 15:08 ` Jens Axboe
2021-09-10 15:32 ` Al Viro
2021-09-10 15:36 ` Jens Axboe
2021-09-10 15:04 ` Jens Axboe
2021-09-10 16:06 ` Jens Axboe
2021-09-10 16:44 ` Linus Torvalds
2021-09-10 16:56 ` Al Viro
2021-09-10 16:58 ` Linus Torvalds
2021-09-10 17:26 ` Jens Axboe
2021-09-10 17:31 ` Linus Torvalds
2021-09-10 17:32 ` Jens Axboe
2021-09-10 18:48 ` Al Viro
2021-09-10 19:04 ` Linus Torvalds
2021-09-10 19:10 ` Linus Torvalds
2021-09-10 19:10 ` Jens Axboe
2021-09-10 17:04 ` Jens Axboe
2021-09-09 22:54 ` Pavel Begunkov
2021-09-09 22:57 ` Pavel Begunkov
2021-09-09 23:14 ` Pavel Begunkov
2021-09-09 20:03 ` pr-tracker-bot
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