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Date:   Tue, 19 Oct 2021 23:41:41 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Pavel Begunkov <asml.silence@gmail.com>
Cc:     linux-block@vger.kernel.org, Jens Axboe <axboe@kernel.dk>
Subject: Re: [PATCH 14/16] block: skip advance when async and not needed
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On Tue, Oct 19, 2021 at 10:24:23PM +0100, Pavel Begunkov wrote:
> Nobody cares about iov iterators state if we return -EIOCBQUEUED, so as
> the we now have __blkdev_direct_IO_async(), which gets pages only once,
> we can skip expensive iov_iter_advance(). It's around 1-2% of all CPU
> spent.

This is a pretty horrible code structure.  If you do want to optimize
this case just call __bio_iov_bvec_set directly from your fast path for
the iov_iter_is_bvec case.  And please add a big fat comment explaining
it and document in the commit why this matters using actual numbers on
a real workload.