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

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Date:   Tue, 24 Jan 2023 21:10:31 +0000
From:   David Howells <dhowells@...hat.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     dhowells@...hat.com, Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        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

John Hubbard <jhubbard@...dia.com> wrote:

> > +	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?

I think it should be okay.  The for-loop at the top just skips over empty
segments, so it doesn't really advance things.  There is an error there,
though: it should be i->bvec++, not i->kvec++ in the loop.

David