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Date:   Fri, 10 Sep 2021 07:42:05 +1000
From:   Dave Chinner <david@fromorbit.com>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        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
Message-ID: <20210909214205.GH1756565@dread.disaster.area>
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On Thu, Sep 09, 2021 at 03:19:56PM -0600, Jens Axboe wrote:
> On 9/9/21 1:37 PM, Linus Torvalds wrote:
> > 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.

You might well be thinking of the problem fixed by commit
883a790a8440 ("xfs: don't allow NOWAIT DIO across extent
boundaries").

This fix was indicative of a whole class of issues with IOCB_NOWAIT
being used for multi-IO operations at the filesystem level and being
applied to each sub-segment of the IO that was constructed, rather
than the IO as a whole. Hence a failure on the second or subsequent
segments could return -EAGAIN (and potentially other errors) to the
caller after the segments we successfully submitted consumed part of
the iov...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com