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Date:   Fri, 10 Sep 2021 15:32:39 +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 Fri, Sep 10, 2021 at 09:08:02AM -0600, Jens Axboe wrote:

> > You actually can cut it down even more - nr_segs + iov remains constant
> > all along, so you could get away with just 3 words here...  I would be
> 
> Mmm, the iov pointer remains constant? Maybe I'm missing your point, but
> the various advance functions are quite happy to increment iter->iov or
> iter->bvec, so we need to restore them. From a quick look, looks like
> iter->nr_segs is modified for advancing too.
> 
> What am I missing?

i->iov + i->nr_segs does not change - the places incrementing the former
will decrement the latter by the same amount.  So it's enough to store
either of those - the other one can be recovered by subtracting the
saved value from the current i->iov + i->nr_segs.