[PATCH v6 07/19] mm: Put readahead pages in cache earlier

[Matthew Wilcox]

On Tue, Feb 18, 2020 at 04:01:43PM -0800, John Hubbard wrote:
> How about this instead? It uses the "for" loop fully and more naturally,
> and is easier to read. And it does the same thing:
> 
> static inline struct page *readahead_page(struct readahead_control *rac)
> {
> 	struct page *page;
> 
> 	if (!rac->_nr_pages)
> 		return NULL;
> 
> 	page = xa_load(&rac->mapping->i_pages, rac->_start);
> 	VM_BUG_ON_PAGE(!PageLocked(page), page);
> 	rac->_batch_count = hpage_nr_pages(page);
> 
> 	return page;
> }
> 
> static inline struct page *readahead_next(struct readahead_control *rac)
> {
> 	rac->_nr_pages -= rac->_batch_count;
> 	rac->_start += rac->_batch_count;
> 
> 	return readahead_page(rac);
> }
> 
> #define readahead_for_each(rac, page)			\
> 	for (page = readahead_page(rac); page != NULL;	\
> 	     page = readahead_page(rac))

I'm assuming you mean 'page = readahead_next(rac)' on that second line.

If you keep reading all the way to the penultimate patch, it won't work
for iomap ... at least not in the same way.