linux-kernel - Re: [PATCH 2/6] iov_iter: new iov_iter_pin_pages*(), for FOLL

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Date:   Sun, 27 Feb 2022 14:57:43 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     jhubbard.send.patches@...il.com, Jan Kara <jack@...e.cz>,
        Christoph Hellwig <hch@...radead.org>,
        Dave Chinner <dchinner@...hat.com>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Theodore Ts'o <tytso@....edu>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Miklos Szeredi <miklos@...redi.hu>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Chaitanya Kulkarni <kch@...dia.com>
Cc:     linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH 2/6] iov_iter: new iov_iter_pin_pages*(), for FOLL_PIN
 pages

> +ssize_t iov_iter_pin_pages(struct iov_iter *i,
> +		   struct page **pages, size_t maxsize, unsigned int maxpages,
> +		   size_t *start)
> +{
> +	size_t len;
> +	int n, res;
> +
> +	if (maxsize > i->count)
> +		maxsize = i->count;
> +	if (!maxsize)
> +		return 0;
> +
> +	WARN_ON_ONCE(!iter_is_iovec(i));
> +
> +	if (likely(iter_is_iovec(i))) {
> +		unsigned int gup_flags = 0;
> +		unsigned long addr;
> +
> +		if (iov_iter_rw(i) != WRITE)
> +			gup_flags |= FOLL_WRITE;
> +		if (i->nofault)
> +			gup_flags |= FOLL_NOFAULT;
> +
> +		addr = first_iovec_segment(i, &len, start, maxsize, maxpages);
> +		n = DIV_ROUND_UP(len, PAGE_SIZE);
> +		res = pin_user_pages_fast(addr, n, gup_flags, pages);
> +		if (unlikely(res <= 0))
> +			return res;
> +		return (res == n ? len : res * PAGE_SIZE) - *start;

Trying to be clever like that just makes the code a lot less readable. I
should not have to reason about a return value. Same in the other
function.

-- 
Jens Axboe