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

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Message-ID: <20201120131352.GO29991@casper.infradead.org>
Date:   Fri, 20 Nov 2020 13:13:52 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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 01:00:37PM +0000, Pavel Begunkov wrote:
> On 20/11/2020 12:39, Matthew Wilcox wrote:
> > On Fri, Nov 20, 2020 at 08:14:29AM +0000, Christoph Hellwig wrote:
> >> On Fri, Nov 20, 2020 at 02:54:57AM +0000, Matthew Wilcox wrote:
> >>> 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?
> >>
> >> Which is most effectively done by stopping to to use *blockdev_direct_IO
> >> and switching to iomap instead :)
> > 
> > But iomap still calls iov_iter_npages().  So maybe we need something like
> > this ...
> 
> Yep, all that are not mutually exclusive optimisations.
> Why `return 1`? It seems to be used later in bio_alloc(nr_pages)

because 0 means "no pages".  It does no harm to allocate one biovec
that we then don't use.

> > -	nr_pages = iov_iter_npages(dio->submit.iter, BIO_MAX_PAGES);
> > +	nr_pages = bio_iov_iter_npages(dio->submit.iter);
> >  	if (nr_pages <= 0) {
            ^^^^^^^^^^^^^

> > -		nr_pages = iov_iter_npages(dio->submit.iter, BIO_MAX_PAGES);
> > +		nr_pages = bio_iov_iter_npages(dio->submit.iter);
> >  		iomap_dio_submit_bio(dio, iomap, bio, pos);
> >  		pos += n;
> >  	} while (nr_pages);
                 ^^^^^^^^