Re: [RFC][PATCH] iov_iter: Add ITER_MAPPING

[David Howells <dhowells@redhat.com>]

Matthew Wilcox <willy@infradead.org> wrote:

> It's perfectly legal to have compound pages in the page cache.  Call
> find_subpage(page, xas.xa_index) unconditionally.

Like this?

#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();						\
	xas_for_each(&xas, page, ULONG_MAX) {				\
		if (xas_retry(&xas, page) || xa_is_value(page)) {	\
			WARN_ON(1);					\
			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;						\
}

Note that the walk is not restartable - and the array is supposed to have been
fully populated by the caller for the range specified - so I've made it print
a warning and end the loop if xas_retry() or xa_is_value() return true (which
takes care of the !page case too).  Possibly I could just leave it to fault in
this case and not check.

If PageHuge(page) is true, I presume I need to support that too.  How do I
find out how big the page is?

David