linux-kernel - Re: [PATCH v2 1/2] iov_iter: optimise iov_iter

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Message-ID: <20201120025457.GM29991@casper.infradead.org>
Date:   Fri, 20 Nov 2020 02:54:57 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     Ming Lei <ming.lei@...hat.com>, linux-fsdevel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] iov_iter: optimise iov_iter_npages for bvec

On Fri, Nov 20, 2020 at 02:25:08AM +0000, Pavel Begunkov wrote:
> On 20/11/2020 02:22, Ming Lei wrote:
> > iov_iter_npages(bvec) still can be improved a bit by the following way:
> 
> Yep, was doing exactly that, +a couple of other places that are in my way.

Are you optimising the right thing here?  Assuming you're looking at
the one in do_blockdev_direct_IO(), wouldn't we be better off figuring
out how to copy the bvecs directly from the iov_iter into the bio
rather than calling dio_bio_add_page() for each page?