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Subject: Re: [git pull] iov_iter fixes
From:   Jens Axboe <axboe@kernel.dk>
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Pavel Begunkov <asml.silence@gmail.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
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Date:   Thu, 9 Sep 2021 15:39:15 -0600
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On 9/9/21 3:19 PM, Jens Axboe wrote:
>> 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 */
> 
> That comment is from me, and it goes back a few years. IIRC, it was the
> iomap or xfs code that I hit this with, but honestly I don't remember
> all the details at this point. I can try and play with it and see if it
> still reproduces.

OK, one that I immediately found is just doing O_DIRECT to a block
device or file on XFS. As pages are mapped and added, the iov_iter is
advanced. If we then go and submit and get -EAGAIN, for example, then we
return with what we mapped already consumed.

-- 
Jens Axboe