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Date:   Fri, 10 Sep 2021 14:42:29 +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 07:57:49AM -0600, Jens Axboe wrote:

> It was just a quick hack, might very well be too eager to go through
> those motions. But pondering this instead of sleeping, we don't need to
> copy all of iov_iter in order to restore the state, and we can use the
> same advance after restoring. So something like this may be more
> palatable. Caveat - again untested, and I haven't tested the performance
> impact of this at all.

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
surprised if extra memory traffic had shown up - it's well within the
noise from register spills, (un)inlining, etc.  We are talking about
3 (or 4, with your variant) extra words on one stack frame (and that'd
be further offset by removal of ->truncated); I'd still like to see the
profiling data, but concerns about extra memory traffic due to that
are, IMO, misplaced.