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Date:   Fri, 10 Sep 2021 03:36:10 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Jens Axboe <axboe@kernel.dk>
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
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On Thu, Sep 09, 2021 at 09:30:03PM -0600, Jens Axboe wrote:

> > 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.

Might very well be true, but... won't your patch hit the reimport on
every short read?  And the cost of uaccess in there is *much* higher
than copying of 48 bytes into local variable...

Or am I misreading your patch?  Note that short reads on reaching
EOF are obviously normal - it's not a rare case at all.