linux-kernel - RE: [PATCH 2/2] block: no-copy bvec for direct IO

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Message-ID: <9e80d0f0adc84cc9995e58e2d6aad580@AcuMS.aculab.com>
Date:   Wed, 9 Dec 2020 21:13:54 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Pavel Begunkov' <asml.silence@...il.com>,
        Jens Axboe <axboe@...nel.dk>
CC:     Alexander Viro <viro@...iv.linux.org.uk>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "io-uring@...r.kernel.org" <io-uring@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Matthew Wilcox <willy@...radead.org>
Subject: RE: [PATCH 2/2] block: no-copy bvec for direct IO

From: Pavel Begunkov
> Sent: 09 December 2020 02:20
> 
> The block layer spends quite a while in blkdev_direct_IO() to copy and
> initialise bio's bvec. However, if we've already got a bvec in the input
> iterator it might be reused in some cases, i.e. when new
> ITER_BVEC_FLAG_FIXED flag is set. Simple tests show considerable
> performance boost, and it also reduces memory footprint.
...
> @@ -398,7 +422,11 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_vecs)
>  		bio->bi_end_io = blkdev_bio_end_io;
>  		bio->bi_ioprio = iocb->ki_ioprio;
> 
> -		ret = bio_iov_iter_get_pages(bio, iter);
> +		if (iov_iter_is_bvec(iter) && iov_iter_bvec_fixed(iter))
> +			ret = bio_iov_fixed_bvec_get_pages(bio, iter);
> +		else
> +			ret = bio_iov_iter_get_pages(bio, iter);
> +

Is it necessary to check iov_iter_is_bvec() as well as iov_iter_bvec_fixed() ?
If so it is probably worth using & not && so the compiler stands
a chance of generating a & (B | C) == B instead of 2 conditionals.
(I think I saw the bits in the same field being tested.

	David

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