On Wed, Sep 8, 2021 at 9:24 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> Fixes for io-uring handling of iov_iter reexpands
Ugh.
I have pulled this, because I understand what it does and I agree it
fixes a bug, but it really feels very very hacky and wrong to me.
It really smells like io-uring is doing a "iov_iter_revert()" using a
number that it pulls incorrectly out of its arse.
So when io-uring does that
iov_iter_revert(iter, io_size - iov_iter_count(iter));
what it *really* wants to do is just basically "iov_iter_reset(iter)".
And that's basically what that addition of that "iov_iter_reexpand()"
tries to effectively do.
Wouldn't it be better to have a function that does exactly that?
Alternatively (and I'm cc'ing Jens) is is not possible for the
io-uring code to know how many bytes it *actually* used, rather than
saying that "ok, the iter originally had X bytes, now it has Y bytes,
so it must have used X-Y bytes" which was actively wrong for the case
where something ended up truncating the IO for some reason.
Because I note that io-uring does that
/* may have left rw->iter inconsistent on -EIOCBQUEUED */
iov_iter_revert(&rw->iter, req->result - iov_iter_count(&rw->iter));
in io_resubmit_prep() too, and that you guys missed that it's the
exact same issue, and needs that exact same iov_iter_reexpand().
That "req->result" is once again the *original* length, and the above
code once again mis-handles the case of "oh, the iov got truncated
because of some IO limit".
So I've pulled this, but I think it is
(a) ugly nasty
(b) incomplete and misses a case
and needs more thought. At the VERY least it needs that
iov_iter_reexpand() in io_resubmit_prep() too, I think.
I'd like the comments expanded too. In particular that
/* some cases will consume bytes even on error returns */
really should expand on the "some cases" thing, and why such an error
isn't fatal buye should be retried asynchronously blindly like this?
Because I think _that_ is part of the fundamental issue here - the
io_uring code tries to just blindly re-submit the whole thing, and it
does it very badly and actually incorrectly.
Or am I missing something?
Linus
