linux-kernel - Re: [RFC][PATCH] iov_iter: Add ITER

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Date:   Fri, 24 Jan 2020 14:58:37 +0000
From:   David Howells <dhowells@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     dhowells@...hat.com, Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] iov_iter: Add ITER_MAPPING

Okay, so this then?

#define iterate_mapping(i, n, __v, skip, STEP) {		\
	struct page *page;					\
	size_t wanted = n, seg, offset;				\
	loff_t start = i->mapping_start + skip;			\
	pgoff_t index = start >> PAGE_SHIFT;			\
								\
	XA_STATE(xas, &i->mapping->i_pages, index);		\
								\
	rcu_read_lock();						\
	for (page = xas_load(&xas); page; page = xas_next(&xas)) {	\
		if (xas_retry(&xas, page))				\
			continue;					\
		if (WARN_ON(xa_is_value(page)))				\
			break;						\
		if (WARN_ON(PageHuge(page)))				\
			break;						\
		__v.bv_page = find_subpage(page, xas.xa_index);		\
		offset = (i->mapping_start + skip) & ~PAGE_MASK;	\
		seg = PAGE_SIZE - offset;			\
		__v.bv_offset = offset;				\
		__v.bv_len = min(n, seg);			\
		(void)(STEP);					\
		n -= __v.bv_len;				\
		skip += __v.bv_len;				\
		if (n == 0)					\
			break;					\
	}							\
	rcu_read_unlock();					\
	n = wanted - n;						\
}

> We could also have an ITER_XARRAY which you just pass &mapping->i_pages
> to.  I don't think you use any other part of the mapping, so that would
> be a more generic version that is equally efficient.

I could give that a go.  Out of interest, are there any other users of xarray
that might use it that don't have a mapping handy?

David