Re: [PATCH v9 2/8] iov_iter: Add a function to extract a page list from an iterator

From: John Hubbard <jhubbard@nvidia.com>
To: David Howells <dhowells@redhat.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Christoph Hellwig <hch@infradead.org>
Cc: Matthew Wilcox <willy@infradead.org>,
	Jens Axboe <axboe@kernel.dk>, "Jan Kara" <jack@suse.cz>,
	Jeff Layton <jlayton@kernel.org>,
	David Hildenbrand <david@redhat.com>,
	Jason Gunthorpe <jgg@nvidia.com>,
	Logan Gunthorpe <logang@deltatee.com>,
	<linux-fsdevel@vger.kernel.org>, <linux-block@vger.kernel.org>,
	<linux-kernel@vger.kernel.org>, "Christoph Hellwig" <hch@lst.de>,
	<linux-mm@kvack.org>
Subject: Re: [PATCH v9 2/8] iov_iter: Add a function to extract a page list from an iterator
Date: Tue, 24 Jan 2023 12:50:18 -0800	[thread overview]
Message-ID: <4be974aa-2beb-9ae5-3f48-7dde6241b0c7@nvidia.com> (raw)
In-Reply-To: <20230124170108.1070389-3-dhowells@redhat.com>

On 1/24/23 09:01, David Howells wrote:
...
> +/*
> + * Extract a list of contiguous pages from an ITER_BVEC iterator.  This does
> + * not get references on the pages, nor does it get a pin on them.
> + */
> +static ssize_t iov_iter_extract_bvec_pages(struct iov_iter *i,
> +					   struct page ***pages, size_t maxsize,
> +					   unsigned int maxpages,
> +					   unsigned int extraction_flags,
> +					   size_t *offset0)
> +{
> +	struct page **p, *page;
> +	size_t skip = i->iov_offset, offset;
> +	int k;
> +
> +	for (;;) {
> +		if (i->nr_segs == 0)
> +			return 0;
> +		maxsize = min(maxsize, i->bvec->bv_len - skip);
> +		if (maxsize)
> +			break;
> +		i->iov_offset = 0;
> +		i->nr_segs--;
> +		i->kvec++;
> +		skip = 0;
> +	}
> +
> +	skip += i->bvec->bv_offset;
> +	page = i->bvec->bv_page + skip / PAGE_SIZE;
> +	offset = skip % PAGE_SIZE;
> +	*offset0 = offset;
> +
> +	maxpages = want_pages_array(pages, maxsize, offset, maxpages);
> +	if (!maxpages)
> +		return -ENOMEM;

Is it OK that the iov_iter position has been advanced, and left that way,
in the case of an early -ENOMEM return here?

thanks,
-- 
John Hubbard
NVIDIA