Re: [PATCH 2/6] iov_iter: new iov_iter_pin_pages*(), for FOLL_PIN pages

From: Jens Axboe <axboe@kernel.dk>
To: jhubbard.send.patches@gmail.com, Jan Kara <jack@suse.cz>,
	Christoph Hellwig <hch@infradead.org>,
	Dave Chinner <dchinner@redhat.com>,
	"Darrick J . Wong" <djwong@kernel.org>,
	Theodore Ts'o <tytso@mit.edu>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Miklos Szeredi <miklos@szeredi.hu>,
	Andrew Morton <akpm@linux-foundation.org>,
	Chaitanya Kulkarni <kch@nvidia.com>
Cc: linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-xfs@vger.kernel.org, linux-mm@kvack.org,
	LKML <linux-kernel@vger.kernel.org>,
	John Hubbard <jhubbard@nvidia.com>
Subject: Re: [PATCH 2/6] iov_iter: new iov_iter_pin_pages*(), for FOLL_PIN pages
Date: Sun, 27 Feb 2022 14:57:43 -0700	[thread overview]
Message-ID: <06469550-a679-145f-b16e-2f1ffc0b07af@kernel.dk> (raw)
In-Reply-To: <20220227093434.2889464-3-jhubbard@nvidia.com>

> +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