linux-kernel - Re: [git pull] iov

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Message-ID: <88f83037-0842-faba-b68f-1d4574fb45cb@kernel.dk>
Date:   Thu, 9 Sep 2021 19:35:13 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Pavel Begunkov <asml.silence@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [git pull] iov_iter fixes

On 9/9/21 4:56 PM, Linus Torvalds wrote:
> On Thu, Sep 9, 2021 at 3:21 PM Jens Axboe <axboe@...nel.dk> wrote:
>>
>> On 9/9/21 3:56 PM, Linus Torvalds wrote:
>>>
>>> IOW, can't we have  that
>>>
>>>         ret = io_iter_do_read(req, iter);
>>>
>>> return partial success - and if XFS does that "update iovec on
>>> failure", I could easily see that same code - or something else -
>>> having done the exact same thing.
>>>
>>> Put another way: if the iovec isn't guaranteed to be coherent when an
>>> actual error occurs, then why would it be guaranteed to be coherent
>>> with a partial success value?
>>>
>>> Because in most cases - I'd argue pretty much all - those "partial
>>> success" cases are *exactly* the same as the error cases, it's just
>>> that we had a loop and one or more iterations succeeded before it hit
>>> the error case.
>>
>> Right, which is why the reset would be nice, but reexpand + revert at
>> least works and accomplishes the same even if it doesn't look as pretty.
> 
> You miss my point.
> 
> The partial success case seems to do the wrong thing.
> 
> Or am I misreading things? Lookie here, in io_read():
> 
>         ret = io_iter_do_read(req, iter);
> 
> let's say that something succeeds partially, does X bytes, and returns
> a positive X.
> 
> The if-statements following it then do not trigger:
> 
>         if (ret == -EAGAIN || (req->flags & REQ_F_REISSUE)) {
>   .. not this case ..
>         } else if (ret == -EIOCBQUEUED) {
>   .. nor this ..
>         } else if (ret <= 0 || ret == io_size || !force_nonblock ||
>                    (req->flags & REQ_F_NOWAIT) || !(req->flags & REQ_F_ISREG)) {
>   .. nor this ..
>         }
> 
> so nothing has been done to the iovec at all.
> 
> Then it does
> 
>         ret2 = io_setup_async_rw(req, iovec, inline_vecs, iter, true);
> 
> using that iovec that has *not* been reset, even though it really
> should have been reset to "X bytes read".
> 
> See what I'm trying to say?

Yep ok I follow you now. And yes, if we get a partial one but one that
has more consumed than what was returned, that would not work well. I'm
guessing that a) we've never seen that, or b) we always end up with
either correctly advanced OR fully advanced, and the fully advanced case
would then just return 0 next time and we'd just get a short IO back to
userspace.

The safer way here would likely be to import the iovec again. We're
still in the context of the original submission, and the sqe hasn't been
consumed in the ring yet, so that can be done safely.

-- 
Jens Axboe