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

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Message-ID: <4be974aa-2beb-9ae5-3f48-7dde6241b0c7@nvidia.com>
Date:   Tue, 24 Jan 2023 12:50:18 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>
CC:     Matthew Wilcox <willy@...radead.org>, Jens Axboe <axboe@...nel.dk>,
        "Jan Kara" <jack@...e.cz>, Jeff Layton <jlayton@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        <linux-fsdevel@...r.kernel.org>, <linux-block@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, "Christoph Hellwig" <hch@....de>,
        <linux-mm@...ck.org>
Subject: Re: [PATCH v9 2/8] iov_iter: Add a function to extract a page list
 from an iterator

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